The Door Problem of Combat Design 2

Let’s say I’m making a level for a classic first person shooter. To start, I build an arena and add some monsters. I don’t want the player to be attacked as soon as they start the game, so I add a hallway to one side of the arena and start the player there.

(Here the “E”s mark enemy monsters in the arena and the “P” is the player start in the hall.)

My level is simple, but I’m happy with what I’ve built, so I invite a friend to playtest. My friend walks along the hallway, enters the arena, and alerts the monsters, all according to plan. Then things go wrong. Instead of fighting in the arena, my friend steps back into the hall and fights from the doorway as the enemies funnel in. Instead of a dynamic gun ballet of dodged projectiles and swirling destruction, my friend has turned my level into a shooting gallery: dull, safe, and slow.

This is a door problem. Specifically, this is the door problem of combat encounter design. This is the problem of drawing players into gameplay spaces and pushing them to play in exciting ways.

If the door is the problem, why not make it bigger, or have the hall open directly to the arena? But the door itself isn’t the problem. The problem is the relationship between these two spaces, a problem that the player experiences when crossing the threshold, which is often a door.

If we widen the doorframe so the hall feeds into the arena, we get another version of the same problem. In some ways it’s better, in other ways worse. Even with the bigger threshold, there is nothing about the arena that draws the player in, nor anything about the hall that pushes the player out.

Before we look at some techniques for solving this problem, let’s explore what’s going wrong and why.

What Am I Really Designing?

First, the fundamentals. When we build a level for a classic first person shooter, what are we making? If we can ignore the thematic and narrative functions of a level, what is the level even about? What are we trying to build when we design a combat encounter?

Building a level for a classic shooter is not about killing scary monsters with cool guns, though this is part of their appeal. A classic shooter level also isn’t about its sequence of locks and keys. These are both means to an end, and that end is map control. As the player moves through a level, they are taking territory from their enemy and locking the level into a solved state.

This fight for territory is one aspect of map control, but there are other aspects to consider. In an abstract sense, map control is about a player developing their options while limiting the options available to their opponent. These options depend on resources, which include map position, but also health, items, and ammo. For example, if the player has health and ammo, they can press an attack through a dangerous space to a stronger position in an exchange of resources. Without their resource of health and ammo, this option doesn’t exist.

Conversely, when the player dies, it is partly because they lost the game of map control and ran out of options. Of course, first person shooters are also about attention, target prioritization, and the player’s ability to internalize gameplay patterns. When the player dies, attrition and imprecision also play a role. But as the player becomes comfortable with these fundamentals, the game becomes increasingly about map control.

For a combat encounter, map control means understanding the relationship between the enemies and the environment and the resources available. For the player, map control means reducing the number of ways for enemies to attack while also developing the number of ways they can attack the enemy.¹

Applying Map Control to The Door Problem

For my level with the door problem, what does map control mean? When my friend first stepped into the arena and alerted the monsters, my friend had a choice:

This choice depends on several unknowns. If the player fights in the arena and enemy reinforcements arrive, then the player may be overwhelmed and should have fought from the door instead. Or, if the player lacks the firepower to kill the monsters as they approach, then the player is out of options and will die in the hallway. This choice depends on the player’s understanding of the game and the conventions of its level design.

We can describe games of map control as graphs of positions, with attention to the positions that the player can attack.

Here the player occupies node 2, controls node 1, and can attack nodes 3 and 4. The enemy occupies node 5 and controls node 6.

Now here’s a simplified version of my level with the door problem:

(“P” marks the player position, and “E”s mark the enemy positions. The “?”s indicate hidden information.)

This abstraction shows how stepping into the arena exposes the player to more angles of attack than they may be able to manage. To successfully fight in this arena, the player would need a deep, internalized understanding of the AI behavior and game mechanics. Instead, most players will fight from the door.

Seeing Through The Player’s Eyes

Another way to diagnose the door problem is with value diagrams. These are a tool to abstract how the player perceives the level, assigns values, and forms plans.²

(Here is a value diagram from Aubrey Serr’s GDC 2019 talk about nonlinear level design.)

To construct a value diagram, we have to consider our game’s mechanics and how the level geometry supports or hinders them. For shooter gameplay, convex corners make strong cover; they afford the opportunity to step out, fire a shot, and step back with minimal risk. Other forms of level geometry, like deadends or killzones, can repel the player because of how they limit the player’s options to move and shoot without taking damage.

For shooter gameplay, the values of the level geometry change as the player and enemies move and fight. The values also change based on the types of enemies and the whole range of gameplay systems. If the player sees a monster that fires bouncing grenades, the player needs to evaluate the level differently; the cover that was strong against hitscan enemies is now more dangerous than the open floor space.

This dynamism invites a kind of play that Matthias Worch called “prioritization choice” in his GDC 2014 talk on “Meaningful Choice for Game Level Design”. Worch defined prioritization choice as “the complex interplay of systems that are easily understood individually, but that combine into situations that don’t have a consistent and obviously superior tactic.” This means the player gets to form opinions about the best plan for solving this dynamic environment.

That is to say, with shooter gameplay, a value diagram describes a moment of the player’s perception and evaluation. In another moment, as the arena shifts, the value diagram may be different, with new threats at different priorities.

If we apply these diagrams to my level with the door problem, we get an idea of what’s going wrong. The whole arena space repels the player with vulnerability and unknowns. But the hallway offers cover where the player can safely attack this group of enemies.

This value diagram tells us what we already knew. But, by thinking of the level in terms of the player’s perception and evaluation, we have a hint at how to solve the door problem. If I want my friend to fight in the arena, I need to change the level geometry so the space is more positive and inviting than fighting from the door.

Techniques for Solving the Door Problem

Now that we have the concept of map control and the tool of value diagrams to understand how the player evaluates a level, let’s look at a few techniques for solving our door problem. This isn’t a comprehensive list, but it should get us started.

Foothold of Cover

The easiest solution is to add a foothold of cover. This is a strong position to draw the player into the arena. From this foothold, the player can push deeper into the arena to fight. The only reason for the player to retreat to the doorway at this point is if the foothold is too exposed or if the enemies can easily surround the foothold and push the player back.

Here our foothold is a simple block that provides full cover. The player can’t see the enemies on the far side of the block or around the edges.

From the perspective of map control, we can see how this foothold of cover limits the angles of attack for the enemies and makes the arena easier for the player to manage.

From the perspective of a value diagram, we can see how this foothold of cover draws the player into the arena.

Reward for Risk

We can also affect the player’s evaluation of a level by adding powerups and items. If we place a strong powerup in the middle of the arena, this may attract the player to risk the danger.

However, once the player has the powerup, there is no longer a reason for staying in the arena, and the player may return to the hall. Depending on the effect of the powerup, this technique is not sufficient to solve the door problem.

Another problem with this technique is that we can only give a powerup to the player every so often. If the player in the hallway is still receiving the benefits of a previous arena’s powerup, then the powerup in the new arena offers little of value.

Hidden Information

In games of map control, one tactic is to divide and conquer. We can encourage this tactic by partitioning the arena into layers. Once our arena is divided, there is no longer a position that can see everything; the player has to keep moving to be aware of their enemies.

The walls that hide the information also function as islands of cover that may attract the player into the arena. As the player alerts the enemies and combat begins, these walls become options for the player to control while fighting.

In a layout like this, information becomes another resource of map control. The player can spend time and positioning to gain information, or they can pay the opportunity cost of information to stay where they are. Even when there are no monsters in a dense, partitioned arena, the player may feel apprehensive about the hidden information and feel drawn into orbiting the space to control that information. In terms of a value diagram, this apprehension is an average negative with pockets of positive space at the cover corners drawing the player in.

Hidden information also discourages the player from retreating. Here, if the player retreats to the hall, they are giving up map control to an unknown group of enemies; there may be more monsters in the arena than the player can survive by fighting from the door. Retreating gives up the options to improvise as the hidden information reveals itself.

AI Leashing

One aspect of the door problem in my level is that the enemies rush toward the player and limit the player’s movement. We can modify the level geometry of our arena to keep the enemies at a distance on islands of territory, like leashing a dog to pole. This means the player has to go on the offense instead of letting the enemies funnel into the door.

With classic shooter levels, the easiest way to leash enemies is with height differences in the floor. Depending on the specifics of the game, the AI may not be able to drop down or climb up from a height greater than a step, which means they are stuck on the island and can only attack. With modern games, we have more tools to affect AI behavior with zones, defense volumes, path weighting, and one-off scripting.³

As with the reward for risk technique, leashing AI is not sufficient to solve the door problem. This is a technique to use in combination with our other techniques for drawing the player into the arena.

One Way Paths

We can also use more forceful techniques to make the player fight in the arena. We can require the player to drop down, or we can close the door behind them. We could have the player enter the room through a one-way elevator, or use a teleport.

For modern games, we could play a cutscene and take the moment to develop our characters and remind the player of their goal while we gate off the previous area of the level. But, for classic first person shooters, there is an expectation for secrets and rewards for exploration. In this older context, one-way paths break conventions and deny player agency. When using this technique in this context, open new routes for backtracking or create loops in the layout.

Putting it All Together

Now that we understand the problem and have some techniques to solve it, let’s put it all together.

Without changing the number of enemies, or drastically increasing the size of the arena, I applied several of the techniques:

Conclusion

With my example level throughout this post, this may sound like a new mapper’s problem. Of course an arena needs cover! Of course we should divide arenas into layers of hidden information!

But this door problem remains relevant. My most recent map for Quake suffered from the door problem, and it wasn’t the only one. With each new level I make, and each new combat encounter I design, I think about how I am drawing the player into the space and how I am addressing the door problem.

As a designer, I want my players to interact with the deeper game of map control. I want to offer prioritization choices and opportunities to form plans and tactics instead of reducing gameplay to a shooting gallery.

I hope this article will help you identify door problems in your own work. And I hope some of the techniques I outlined will prove useful in overcoming those problems.

Thank you for reading,
-Andrew

Footnotes and Further Reading

[1] If you want to understand the concept of map control better, I recommend watching the Rapha versus Cooller match in Quake Live: https://youtu.be/XdkDjsBiO58?t=155. What appears to be a game of reflexes, attention, and precision, becomes a game of tactics as well. Although this is a multiplayer example, the concept of map control still applies. The main difference is that we design singleplayer combat encounters to be solved, similar to what chess problems are for chess.

[2] The concept for value diagrams began with Randy Smith’s GDC 2006 talk “Level Building for Stealth Games”. I first learned about these diagrams as “valence theory” in Robert Yang’s “Dark Pasts (Part 4)” analyzing the level design of immersive sims. More recently, Aubrey Serr adapted Smith’s diagrams to action game design in his GDC 2019 talk “Radically Nonlinear Level Design”.

For more thoughts on how player perception affects their movement into an arena, check out Blake Rebouche’s GDC 2018 talk “Balancing Action and RPG in Horizon Zero Dawn Quests”. Especially of interest is about 15 minutes in, when he describes the bunker sections of the game.

[3] For more on these modern techniques for AI combat encounters, check out Matthew Gallant’s GDC 2017 talk on authored and systemic AI in Uncharted 4.

The original version of this article was posted at https://andrewyoderdesign.blog/2019/08/04/the-door-problem-of-combat-design on August 4th, 2019.