Flow
How players move through the level
In level design, flow is the generalized player movement pattern through your level. Flow is more about the feel, sensation, and smoothness of moving around the level.
Fast-paced arcade-style games might favor a smooth flow with wide generous turns and minimal interruptions. Meanwhile a horror game or walking simulator might work better with sharp narrow turns, blind intersections, S-shaped curves, and maze-like dead ends to emphasize exploration.
Do not confuse this spatial theory of flow with flow (game design), the psychological theory that players maintain a highly-engaged "flow state" when game difficulty and player skill are in equilibrium. Flow (level design) is about how it feels to move through the level.

Types of flow

Different layouts lead to different lines of movement. Compare the two race tracks below; what skills do they test of their players?
track maps with numbered turns for Spa-Francorchamps (F1) vs. Daytona (NASCAR), by Will Pittenger (under CC-BY-SA license)
The Circuit de Spa-Francorchamps (left) is a European-style Formula One (F1) racing circuit with a wide variety of turns, sightlines, and slopes. Drivers at the Belgian Grand Prix must flawlessly perform a sprint of 44 laps. In contrast, the Daytona Speedway (right) is a US-style NASCAR race track that is basically a flat wide oval with banked turns. Drivers at the Daytona 500 focus less on negotiating terrain, and more on close quarters bump-and-grinds in a crowd of cars, where small variations result in bigger consequences (and more catastrophic wrecks) that ripple across a marathon of 200 laps. Both of these "levels" serve the same core mechanic of motor racing, but emphasize very different patterns of movement, and thus very different experiences and strategies.
But we're not here to legislate a Formula One vs NASCAR debate. Both are equally boring interesting. Likewise, every game ideally has multiple types of map flows, complex and simple. Quake 3 Arena had both The Camping Grounds and The Longest Yard, Counter-Strike had both de_dust2 and fy_iceworld, Call of Duty Modern Warfare had both Crash and Shipment. Multiple flavors of flow feel richer.

Critical path

The critical path (or golden path) is the minimum viable path to complete the level and progress through the game. More generally, it represents an idealized player flow that ignores optional side areas. It highlights the most important ("critical"!) parts of the level.
Critical / golden path does not seek to predict actual player behavior because most players will wander, explore, linger, or even just get confused. A critical path conveys your ideal design goals and requirements, but not the reality.
If you are seeking feedback or collaborating in a team, the critical path conveys the layout's functional logic and design intent to someone else. It makes your layout much easier to understand because it helps communicate what is most important about it.
Single player layouts need some sort of labeled critical path. Sketch, highlight, or mark the critical path in the drawing -- or if the result would be visually busy, at least mark the player start and exit(s). Include occasional arrows to indicate the player's direction of travel, and number or label any notable setpieces, encounters, puzzles, or areas.
example level layout with marked critical path and numbered gameplay beats

Critical path as scoping tool

In engineering, project managers prioritize tasks to find the most efficient way to work. This critical path method (CPM) is about listing all necessary tasks and tracing dependencies. Which task must come first and why? Charting the critical path helps them understand the logic and workflow for the project.
We can use critical paths for level design in a similar way. How important is each part of the level, who will see it? If it's on the critical path, then all players will see it. But if it's not on the critical path then only some players will see it, so then maybe it's less important to finish immediately.
The critical path helps us imagine what a "minimal viable" layout looks like, and it can help the level designer decide how to delegate tasks to others, or even scope down by cutting that extraneous content entirely.
the yellow hatched area could be removed without affecting the critical path
what if we cut ALL "unnecessary" areas? now it's less work to build but does it still fulfill our experience goals?

Against critical paths

Designing your layout around a critical path is potentially a reductive way for thinking about the player's relationship to virtual space. Critical paths instrumentalize the world in terms of resources, gates, and objectives, a checklist of activities imposed on the player by a level designer. Can games and levels be more than just "content" for users to consume? Sometimes the answer is yes.
What if your project requires a high degree of realism, plausibility, or sense of place? Building your level around a critical path usually results in a more linear, controlled, and "video game-y" feeling space. Instead, some designers prefer to assemble a critical path after an already-complete layout and blockout, to flow around existing level geometry.
Martin Hollis, producer of Goldeneye 007 (1997) for Nintendo 64 explains their level design process:
"The level creators, or architects were working without much level design, by which I mean often they had no player start points or exits in mind. Certainly they didn’t think about enemy positions or object positions. Their job was simply to produce an interesting space. After the levels were made, Dave or sometimes Duncan would be faced with filling them with objectives, enemies, and stuff. The benefit of this sloppy unplanned approach was that many of the levels in the game have a realistic and non-linear feel. There are rooms with no direct relevance to the level. There are multiple routes across the level. This is an anti-game design approach, frankly. It is inefficient because much of the level is unnecessary to the gameplay. But it contributes to a greater sense of freedom, and also realism. And in turn this sense of freedom and realism contributed enormously to the success of the game.” -- Martin Hollis, producer of Goldeneye 007 (N64) as quoted in "Anti-Design / Backwards Game Design in Goldeneye 007" by Chris DeLeon
level geometry for "Surface" in Goldeneye 007 (1997) captured by Chris DeLeon https://imgur.com/a/MY59R

Circulation

In architecture, circulation refers to the areas that connect other areas, like hallways, aisles, and stairs. Some level designers might call this connectivity but for this book we think circulation is a better way to think about this.
Thinking in terms of circulation offers a useful compromise between a critical path method vs. a build-first method. We're able to imply some circulation is more important than other circulation (like a critical path) without prescribing a specific path to follow (like the build-first method and more like real-life). Effective circulation design aids wayfinding, the player's ability to navigate surroundings to reach their intended destination.
Architects distinguish between several types of circulation:
  • Primary vs. secondary circulation (path to main elevator vs. path to storage closet)
  • Public vs. private circulation (ground floor lobby vs. HVAC maintenance corridor)
  • Horizontal vs. vertical circulation (hallway vs. stairs)
  • Pedestrian vs vehicle (sidewalk vs street)
Circulation diagram for Achievement Preparatory Academy middle school by Studio Twenty Seven Architecture
Players follow circulation based on their current objectives. If they must find the exit, then players will follow primary circulation. If they need to reach the roof, players will look for vertical circulation. If players need to sneak around or explore for resources, they use secondary circulation.
Generally, wider bigger passages feel like primary public circulation, while narrower areas will feel more like secondary circulation. But imagine walking in a level and seeing a big wide door and a smaller narrower door. Which door leads to the exit? The big door could lead to primary circulation and an exit. But if the bigger door looks like a garage door for a vehicle, then maybe it's locked and we should take the smaller door... so now the smaller door is more likely to feel like primary circulation...
In level design there's a difference between what looks/feels like circulation vs. what functions like circulation. In this book, we will call this diegetic circulation and formal circulation.

Diegetic circulation

In film studies, the "diegesis" is the imaginary fictional world depicted on screen. Diegetic circulation refers to how fictional in-game characters would use the space. Ask yourself, how would players roleplay their movement through the level? What is the theme of this space? What is the story behind this circulation pattern?
For single player levels, conveying circulation is crucial for narrative. If you want your level to feel like a plausible real-world space, then you must also think like a plausible real-world architect with plausible real-world circulation.
When designing their exploration game Gone Home (2013), The Fullbright Company chose to work with an old mansion theme because modern suburban houses orbit around a central room with flat dense connectivity, but mansions branch deeply into distinct wings. This mansion trope supported the hub-and-spoke typology they wanted, which helped them tell the story they wanted.
Gone Home (2013) layout with critical path overlay (in green); from "The Level Design of Gone Home" by Steve Gaynor and Kate Craig, GDC 2015
The first half of Gone Home's critical path starts from the "Front Porch" (bottom center) and ends in the Basement (top left) while frequently backtracking on itself, resulting in a free-form exploration feel that supports Gone Home's experience goal of investigating an empty mansion.
It wouldn't feel like an investigation (nor a mansion) if every clue led to a new area. Instead, we learn to dwell in this space and grow more familiar with it. In this way, architectural research is crucial for levels focused on story, otherwise the diegetic circulation may not support the desired critical path.

Formal circulation

In art history, "formal" refers to the form, the shape and structure of an artwork. Formal circulation focuses on the layout's physical function and affordance for movement, and de-emphasizes the history and cultural context. We ignore the theme and setting, and study abstract level geometry. Ask yourself, how would a professional player or speedrunner understand this map?
Primary formal circulation in a multiplayer map is called a lane. Lanes help players predict and coordinate movement. A large map with too many lanes or no clear lane hierarchy will function like a maze; players won't know where to focus their efforts, get lost, and miss each other. For this reason, most maps use only 1-3 lanes.

One lane

Payload maps for team-based multiplayer games like Team Fortress 2 and Overwatch focus gameplay and conflict along a single lane that snakes around the entire map. The attacking team must "push" the payload cart forward by standing near it, while the defending team must prevent the cart from reaching the end of the lane. The payload route is prominently marked so that players can easily coordinate efforts (or prepare ambushes). Occasionally, there are extra half-lanes and back alleys to help attackers break through defenses.
one lane typology in payload map "Badwater Basin" for Team Fortress 2

Three lane

Lanes are also common in competitive multiplayer games about territory control such as in MOBAs like League of Legends or military shooters like Call of Duty and Counter-Strike. These games' maps often use a three lane format, three bidirectional critical paths that occasionally branch and intersect via smaller lanes. (In MOBAs, the tangle of smaller interstitial lanes / secondary circulation are collectively known as the jungle.)
three lane typology in Summoner's Rift (League of Legends) and de_dust2 (Counter-Strike: Global Offensive)
If an opponent has blocked progress on one lane, then the other player(s) can attempt to flank around them by advancing on another lane. Alternatively, if a player wants to try to avoid conflict entirely, then they can avoid the main lanes and try to farm the jungle.
Lane asymmetry is important, each lane should feel different. In Summoner's Rift (League of Legends), the top and bottom lanes are longer than the middle lane, and the top lane has a powerful boss while the bottom has a weaker mid boss. In de_dust2 (CS:GO), teams must decide whether to focus on site A or B and how to approach or flank; or if the team chooses poorly, they might need to move through the mid lane to the other side of the map.
Rotating (primarily in Counter-Strike) is when players must move from one lane to another. It generally requires teamwork and coordination, and the tension between rotating vs. not-rotating is a core element of team-based competitive multiplayer games.

Verticality

Verticality is vertical flow, how it feels to move upwards and downwards.
Imagine steadily climbing up a cliff or descending deep into a dark dungeon. These height changes embody the player's progression, and help orient the player towards their goals. A quest might tell the player to travel north, but that goal is impossible if they can't tell where north is. Instead, if the quest tasks the player with climbing to the top of a mountain, or descending down to the bottom of a valley -- up and down are much more obvious and unmistakable.
Our guidelines for vertical flow rely on two game rules common to most 3D games: (1) gravity pulls the player downwards, and (2) the player camera rarely "rolls", which maintains a stable orientation for the sky and horizon. If you change any of these assumptions, then your use of verticality will differ.

Z-levels / floor planes

When planning the verticality in a level, try to chunk it together into floor planes and merge minor height changes into a single floor. Don't try to hold 10 different distinct overlapping layers of floorplans in your mind, because players probably won't be able to process that much complexity either.
Most maps tend to max-out at three different floor planes for any given area. Why three? Much like three lane typology, three plane format consists of a bottom, middle, and top layer. Comparatively, a fourth (p)lane doesn't add new dynamics, because it would simply yet add another middle layer or path.
Note that old school level designers call these Z-levels because in 3DS Max, Doom, Quake, and Unreal, the third dimension (the Z-axis) corresponds to height. But because many game engines and 3D tools use the Y-axis for height, this doesn't really make sense anymore, so in this book we prefer "floor plane" instead.
(image demonstrating floor planes)

Downward flow

With gravity, it is easier to drop down than to climb up. Downward flow is heavier than upward flow.
The most common use of downward flow in level design is the one-way drop. When the player drops down from a ledge, they cannot backtrack and have no choice but to move forward.
Puzzle exploration games use this technique to limit how much space the player must consider at a time, it's a way of saying "everything before this drop is irrelevant to the next challenge." Shooters rely on this pattern heavily to force the player into a combat encounter, or else the player can simply backtrack to pull enemies back through a chokepoint.
(image showing one way drop)

Upward flow

To push against gravity and create more upward flow, you'll have to work hard to create more opportunities to go upwards. Stairs, ramps, and ladders are all common tools to facilitate verticality, but require you to reserve enough space to accommodate them. (Elevators are less common due to their scripting complexity, especially for multi-floor elevators.)
The Quake 3 Arena (1999) multiplayer space floater map Q3DM17 "The Longest Yard" famously used a dozen jump pads and teleporters to create strong upward flow. Because there is very little cover and limited floor area, players must dodge gunfire by flying through the air, while being careful not to fall to their deaths. The designer Brandon James balanced the verticality with the far platform pictured in the lower-right; here, snipers can easily dodge incoming rockets while sniping all the players taking predictable flight arcs.
diagram of jump pad and teleporter trajectories in Q3DM17 "The Longest Yard" by Brandon James for Quake 3 Arena (1999)

Verticality for console shooters

Shooter games on consoles face unique level design challenges relating to input. Because console gamers use gamepads with analog sticks, the fixed turning rate from the analog stick makes it much more difficult to rapidly turn and track targets. In contrast, a mouse offers much more acceleration and precision. But we're not here to litigate a gamer spat over which input is better -- we're just here to say, if you anticipate your level will have a gamepad-using audience, then you must design accordingly.
For single player levels, that means spawning enemies behind the player ("backspawning") feels extra unfair because the player won't be able to turn around 180 degrees very quickly, and also flying enemy AI must maintain moderate distance and remain at a stable height.
For multiplayer console shooters like Halo or Call of Duty, players often park their camera's vertical rotation aimed at roughly head / chest height, and restrict their aiming movements to the horizontal axis. Levels for these games rarely incorporate very tall height changes, if ever, because it is so taxing and disadvantageous to force players to look very high or very low.
This type of shooter gamepad input only feels good if the level design supports it: lots of wide open landscapes with shallow slopes, and discrete floor planes that remain relatively flat. The left example below has high frequency height variation which will frustrate gamepad users, while the right example uses smoother slopes and flatter floor planes.
(image)

Verticality mechanics

As with everything in level design, verticality depends heavily on your game's core mechanics.
For example, in Counter-Strike, competitive players tune their mouse sensitivity very low because precision matters more than reaction. Subsequently, constant height changes will frustrate their aim, so most CS levels are relatively flat.
That said, CS features a stacking / boosting mechanic where teammates can jump on top of each other to see over obstacles or reach otherwise inaccessible places, thus rewarding team planning and coordination. Vertical flow can be situational and strategic.
an absurd example of players stacking and boosting each other in Counter-Strike 1.5, from GDC 2015: Community Level Design for Competitive CS:GO

How to design flow

After years of observing playtests and analyzing player psychology, experienced level designers can visualize flow in their mind and predict probable player behavior with (misguided?) confidence. But if you haven't developed that intuition yet, then you will benefit from directly visualizing flow.
  • Plot the critical path(s) and where you think most players will go.
  • Highlight primary and secondary circulation.
At any given point in your layout drawing, you should be able to hypothesize: what would the player be thinking here? What are they concerned about? Where are they hopefully trying to go?
Then, when you playtest, you'll see just how often the player deviates from your plans.

Visualize circulation and implied spaces

For each area or room, draw the shortest possible line that connects each exit to another.
Draw lines to connect all major entrances and exits, and draw lines aligned with the dominant axis of every room and hallway.
How to handle cover placement.
Medium

Visualize desire lines

In urban planning, a desire line (or desire path) is a route marked by erosion from foot traffic, which indicates the shortest / most efficient path, even if it does not follow the "official" path.
This type of path is user-created and not architect-created, although some designers improve user-friendliness by paving desire paths into official paths. Nonetheless this technical dynamic is rare in level design because video game environments are rarely deformable, though some recent games like Death Stranding (2019) do feature dynamic support for visualizing user-generated desire paths.
desire paths deviating from sidewalk / cutting across a lawn, photo by Duncan Rawlinson
But even if we don't directly simulate desire paths in games, we can still use this concept as a planning tool in level design. To assess flow, imagine a desire line between the player's position and their destination, and then compare that desire line against the actual critical path / circulation permitted by the layout. What is the simplest possible ideal path the player would like to take? What is the actual path that your level geometry permits?
In the diagram below, the player's desire line (yellow) points to the exit on the second floor, but the flow (red) forces the player onto the stairs. The switchback stairs (left) result in interrupted flow because the player must turn away from their destination to approach the stairs, rapidly change their direction of movement on the landing, and make several sharp 180 degree turns. The relatively straightforward flow (right) follows the structure of the room and the player can climb while mostly facing their destination with only a single 90 degree turn, which feels much smoother.
diagram comparing two possible stairs arrangements with very different flows; level geometry by Andrew Yoder

"Bad flow" is good for single player

It is tempting to say that the zigzag switchback above is "bad flow" that interrupts the line of movement, and therefore must never be used. But sometimes slower flow is desirable. The player must put more effort into maneuvering, and sometimes that's what you want, especially if you want to evoke an experience goal that requires slower movement, or if you want to make a particular encounter more difficult or complex to navigate.
Different flow speeds are vital for pacing. A fast flow will feel faster if you bookend it with slower flow before and after, and vice versa, a slow contemplative section of the level will benefit from faster sequences before and after. Contrasting flow helps differentiate each part of your level, making it more memorable and compelling.
flow analysis of Half-Life 2: Episode Two by Andrew Yoder https://twitter.com/Mclogenog/status/1262007255966367751
In the example above from Half-Life 2: Episode Two, the player must repeatedly pause to drop down through openings in the floor, crawl through obstacles, break through planks and clutter, fight a monster or two in a confined area, and spiral down this tall central room. This type of vertical zigzag "gyre" flow encourages a halting tentative feel, as players will hesitate to drop down without first looking down to see what awaits them. It's not a straightforward line to the exit, and the exit isn't even visible, it has "bad" flow... which is the point. This is a slower section where the flow creates a casual exploration puzzle.
This type of flow could easily feel confusing and disorienting for many players. That's why there are so many one-way drops: it divides this area into four distinct parts and prevents backtracking. Even if the player becomes lost, each individual part is small enough that the player can still figure out the next part of the critical path without trouble.

To review...

Flow is about how players move around the level. We can think about flow in many ways:
  • Critical path: idealized player path to complete the level
  • Circulation: connective areas of the level, both from diegetic (fictive storyworld) and formal (geometric shape) perspectives
  • Lane: primary circulation across competitive multiplayer maps, usually 1-3 lanes
  • Verticality: vertical flow, how easily players can move up or down.
  • Floor plane: the vertical divisions of walkable areas, usually 1-3 floor planes

Further reading

Last modified 2mo ago