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Index:
Creating a Single-Player Combat Space Part 1
Creating a Single-Player Combat Space Part 2
Creating a Single-Player Combat Space Part 3
This article is the third installment in a three-part article that looks at the considerations for creating a single-player combat space, using a walkthrough of the first battle in ‘Killzone Mercenary’ as a working example.
Restricting flanking opportunities for the enemy
The distances in cover were also arranged so that the enemies didn’t have a route leading behind the player. All the cover pieces were arranged so that the assault troopers could move freely between them. The only restriction was that the route from the cover point closest to the climbable pipe to the next piece of cover on the higher floor was much longer than 10 meters. This meant that the enemies would never flank the player from behind if the player stayed on the higher floor. This was good for this early stage of the level as the increase in numbers of enemies would usually be tougher to fight, but as they were restricted to the central area below the player they didn’t pose much of an extra threat.
Giving the enemy flanking opportunities creates a much harder situation to deal with, which is why this wasn’t included in the opening encounter. However, if the player chooses the drop down from the balcony into the arena below, the arrangement of cover from this direction allows for flanking by both the player and the enemy. The rectangular layout of the cover positions means there is a circular movement around the outside of the central kill zone and how the player moves depends on which side the enemy can potentially flank from.
If the player moves to the left corner, for example, the metrics of the assault trooper’s movement would encourage them to move away from this corner and take up positions near the far right corner. The number of cover positions versus the number of total enemies would mean that at least one enemy would have to take a position in the close right corner, allowing them to ‘enfilade’ the player. Enfilade is a military term that refers to flanking an enemy so they are positioned to have no cover from the side and are thus exposed and vulnerable. If this happens here, the player would be forced to move around the edges of the kill zone to take up another position that has a front perpendicular to that which they had been using immediately before, ensuring the player has to move around the environment and cannot stay put in one place. This is a good thing from a design point of view as it ensures the player cannot complete the battle from one position and thus it would not be a static and predictable encounter for them.

The circular navigability around the central killzone
Last Backup Troopers
The last backup wave for this combat encounter is two assault troopers that run into the environment from a door on the ground floor to replenish the enemy count when it drops to two. This is so that the player has to manage a larger enemy count for a longer duration than if it was only replenished once it had dropped to zero. This means that the difficulty is kept up to teach the player to deal with crowds of enemies, as well as stopping the battle from becoming too easy halfway through. It is more obvious to the player when new enemies are added after the current wave is dead, which can be jarring to their immersion as it feels very ‘gamey’. It is also more realistic to introduce enemies in pairs rather than individually as it gives the idea that they are working as a team. Furthermore, the ‘Killzone’ animation system was very sophisticated and if enemies were near each other when they acquired the player as a target they would gesture to each other where the player’s location was. This meant that two troopers entering the arena would appear to communicate tactics to each other, which is not only a much more appealing enemy entrance, but also helps to make the enemies seem intelligent.
The door they enter through is on the side of the map that the final exit is on; the back of the arena, or screen left to the player as they look down from the balcony. It wouldn’t make narrative sense for enemies to enter from the opposite side of the map as this stairwell was locked in the opening cut-scene. It also wouldn’t have made sense in terms of directing the focus as this would draw the attention away from the side of the map that the exit is on, which is not ideal as this is the last part of the fight. In addition, the player has always been fighting towards that direction in order to escape the building, so now to be fighting away from that would be confusing.
The arrangement of the cover on the ground floor already supported the two additional troopers as they entered close to where the very first set of troopers landed. The width of the available cover here also allows the enemy to face the player’s new position. As the player is much further than 15m away whilst they’re on the balcony, the troopers can freely move around between cover positions once they’d arrived.
The arrangement also needed to support a view to the door opening and not obscure the player from seeing the new arrivals. During development the position of the door was adjusted in combination with the position of the pillars for high cover, whilst maintaining architectural continuity as they were part of the support structure.

The final backup troopers arrive through a door that’s visible from the balcony
Last Composition
The last metric we needed to consider was the composition and arrangement of the exit point. It was set at a height of 4 meters so that it supported the vault down animation height for the troopers that entered the environment from here, as well as supporting the step-up animation where the buddy character helps the player up. This final animation was used as a ‘hard gate’ for this area, which is a point in the level where once you progress past it you can no longer go back to this section. It also required the buddy to be in this position before the player could leave, meaning the player couldn’t just rush through the environment to the exit.
The exit also used negative space to indicate that this was the exit in contrast to the surrounding walls. The space framed the sunlight and skybox, also using the lighter colour and tones to draw the player’s eye. Being 4 meters above the floor level it also gave the player the feeling of working upwards towards something; a sense of progression.

The buddy takes position at the exit of the arena in order to boost the player up
Summary
When you design and create the formation of a combat encounter you have to create the layout based on consideration of the NPC types you are using. The metrics of these NPCs will determine how they move around and use the environment, so skillful placement of cover positions and other interactive geometry can influence how the enemies behave and thus how an encounter unfolds. This method of constructing a combat encounter is preferable over that of ‘area restricting’ enemies where possible, as relying too heavily on the latter can lead to exploits by the player and worse still, can lead to the AI not behaving correctly.
A carefully considered level layout can also affect the difficulty without the need for altering an enemy’s default numerics, such as health, damage or accuracy. For example, only providing low cover for the enemy when the player has a height advantage will mean the player can target all the enemies at any time without having to wait for them to reveal themselves. Using high cover for enemies to run between means the player has more chance of losing sight of them thus making it harder. However, if the enemy has nowhere to run to (thanks to a cleverly designed layout) and their standard behavior of poking out of cover is to reveal their full body to the player, then this would actually make it less difficult.
It is also not necessary to include every element of combat in every single encounter. Removing elements is important when you are trying to balance difficulty, for example, removing the opportunity for enemies to flank the player early on in the game means they have fewer things to worry about and consider when learning the game’s combat mechanics. These elements can be layered in at later stages of the game to introduce new encounter styles and to increase the difficulty as the player increases their skill. Removing elements also helps to dictate how you intend an encounter to be played or what narrative element you want the player to experience, such as the player being overrun with enemies and feeling desperate, or them having the upper hand and being in control.
You also don’t want the player to stay static during an encounter, as this will make it repetitive and predictable. It is good to move the player around the environment to experience the combat in different ways, be it by moving the combat focus and direction the player fights towards, or by introducing flanking enemies that enfilade the player and force them to move. Moving the front and the combat focus can also help with navigating the player through the environment in the sequence you intended and help orientate them to be facing the exit or next objective when the battle ends.
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Copyright ©Peter Ellis 2016. Killzone™ Mercenary is the property of Sony Computer Entertainment © 2013. Killzone is a trademark of Sony Entertainment Europe. Killzone: Mercenary is a trademark of Sony Computer Entertainment America LLC.