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Alien Swarm: Reactive Drop - Combat Encounters

When designing combat encounters for Research 7, I looked to the original Alien Swarm for inspiration. Valve had achieved highly engaging combat using the same tools I had available, so it made sense to base my own combat encounters on what Valve did. Research into their combat scenarios showed they took three typical approaches:

  • Drip fed combat – Enemies sent at the player in ones and twos while they’re making their way from A to B, designed to keep the player on their toes, making movement through otherwise empty areas more engaging.

  • Overwhelming numbers – Huge numbers of aliens attack from one direction. These scenarios typically need all team members firing in the same direction to overcome.

  • Multi-directional ambush – Swarms of aliens come from two or more directions, forcing players to split their attention.

These second two types of scenario sometimes give players time to prepare, setting up turrets and mines before a specific action from the player(s) triggers the swarm. Further diversifying these scenarios is the inclusion of multiple enemy types serving different purposes in combat:

  • Drones – The most plentiful aliens. Die quickly but will overwhelm the player in numbers. Likely the cause of most player damage. They only attack physically, so shouldn't be allowed to get close.

  • Rangers & Mortarbugs – Fire projectiles that must be avoided, forcing players to dodge and move. During these manoeuvres, players often lose focus on drones, making combat more challenging and engaging. These aliens effectively zone the battlefield.

  • Shieldbugs & Boomers – Tankier enemies that are more dangerous the closer they get. These need to be focused down by the players, again distracting and giving drones the opportunity to attack.

I designed combat encounters in Research 7 with these techniques and tools in mind.


Level 1

Two encounters require players to defend themselves while security doors unlock. Both encounters feature enemies mainly coming from a single direction. The inclusion of Rangers firing from afar forces the player to dodge, breaking up the players’ defensive formation, forcing them to think on their feet. The inclusion of a Boomer in the final encounter adds tension, as the players have their backs to the wall and can’t back away from it.


Left - Drones swarm from the bottom left section of the screen and are funnelled towards players by a cliff-wall on the right.​ Right - Drones and boomers advance from the bottom of the screen. Boomers are deadlier the closer they get, so must be focused on.


Level 2

An encounter early on has drones attacking from multiple directions, climbing in through windows and emerging through hatches in the floor. Later in this level is a second multi-directional ambush, one which players have the opportunity to prepare for. Boomers again disrupt defensive lines.


Left - Players are ambushed. Drones come through this central window, the window at the left edge of the screen, and through the floor hatch on the far right of the screen.

Right - Another ambush, but one the players have had time to prepare for. Drones climb over the railing at several points, while Rangers fire from afar to ensure the players don't get too comfortable with their backs to the wall.

Level 3

Players remain on the move throughout most of this level and face mainly drip-fed drones, but an encounter in the mines early on requires players to defend themselves from assault from three directions. After the bridge midway through the map, players are rushed by a massive swarm from a single direction, forcing all players to focus.


Left - After triggering an explosion, players are attacked from three directions: top left, bottom left, and right.

Right - Shortly after exiting a cave, an overwhelming number of aliens attack from one direction.


Level 4

Players are attacked from the north on the long bridge to the elevator, with boomers tanking fire and drones seeking to get in close to deal damage. Players need to focus down the boomers without losing track of the drones. A mortar bug fires from the north, further disrupting players' plans.


Players open a set of blast doors and are beset by drones and boomers from the same direction. The boomers are already extremely close when the doors open, so they need to focus them down quickly.

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