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Alien Swarm: Reactive Drop - Game Design

My main objective in creating the Research 7 campaign was to provide players with a fresh, new Alien Swarm experience. The same frantic, impactful, bug-squashing combat but with new challenges, new objectives, new environments, and new enemies.


Alongside the original Alien Swarm, Valve released an Alien Swarm SDK (Software Development Kit), so I loaded it up to see what assets and tools were available for me to create with.


New Environment & Story

On delving into Alien Swarm’s files, I was pleasantly surprised to find several folders of unused art assets. The original Alien Swarm’s campaign featured sci-fi-themed environments, snowy outdoor sections, and the occasional cave-like area, but I found unused grass and dirt textures, lava and water textures, rain and lightning effects, sounds, foliage meshes, and more.


A new forest biome

I’d previously been contemplating how to get my hands on new assets, but using these as a starting point seemed like low-hanging fruit. I began examining the story of the original campaign to find inspiration for a new campaign in a more earthy, grounded setting, and discovered an obscure reference to a shipment of alien samples almost sent to Earth. That shipment was caught at the last moment, but it got me wondering… what if it wasn’t the only one? Perhaps there was a second shipment that got away undetected and made it to Earth, unleashing the alien swarm on an unsuspecting research facility. The more I thought about this idea, the more I liked it. It meant I could combine a narrative inspired by the original Alien Swarm with art assets that were already available to me to create a new story in a fresh environment.


A new lava tunnel biome

New Enemies

While delving into the SDK looking for art assets to use, I stumbled across an unused enemy: the Alien Queen. I placed it into a test level and quickly discovered it had incomplete AI, getting stuck on geometry and frequently ignoring the player. It also had so much health that even the most powerful loadouts would take ages to kill it, and it spawned no gibbed pieces on death, giving players no visual reward for finally besting it. By this point, however, I’d learned enough about some of Hammer’s AI scripting tools and built-in functionality to know that the Queen’s broken AI, poor balance, and lack of impact wasn’t a total dead end. I began experimenting, and found that while the Queen’s AI couldn’t be trusted to operate on its own, I could use the Queen to create a scripted sequence. I spawned the Queen behind shut doors, then instructed it to move in a straight line from A to B once the doors were opened, shepherding players in front of it. The Queen couldn’t attack while moving, so I attached a kill volume to the front of her that would ‘trample’ any players that got too close, adding screen shake as she walked to really sell the effect. I made the Queen pause in places, adding sequences where she’d swipe debris out of her path, and adding an optional sequence where players can shut giant security doors in front of her, which she’ll then bash her way through.

The refreshed Queen enemy chases players along a walkway

The sequence ends with the Queen arriving in front of a mining laser. Her scripting then begins looping, alternating between melee and ranged attacks while players run around the mining laser room fending off alien drones and completing objectives to power up the laser. The mining laser was created to get around the issue of the Queen’s extreme health pool. Interacting with terminals around the laser room activates the laser, which deals enough damage to kill the Queen in seconds. I added gibs and spawned a fountain of grubs where she died to ‘congratulate’ the player for killing her. The scripted sequence certainly isn’t perfect, and I’d do it differently if I had my time again, but I’m largely pleased with how it turned out. It combines the fast-paced gameplay with a new enemy type to provide even long-time Alien Swarm players with a fresh, new experience.

New Objectives

As well as new enemies and environments, I wanted to implement new objectives. Killing the Queen was one of these, but another comes at the end of mission 3, where players are tasked with defending a generator that’s raising a lift they need to escape on. If the generator takes too much damage, the mission is failed.


This functionality didn’t exist in the original campaign, so I had to get creative. I created alien spawners that would instruct swarms of alien drones to ignore players and instead charge at the generator. Once they reached the generator, they’d play a melee attack animation. If that animation was allowed to complete, it added 1 to the generator damage total. If the total hit a threshold, the mission would be failed. I added red and green lights to the front of the generator to show the level of damage, giving players an indication of whether it was about to blow.


If the generator survives, the lift reaches the top and provides players with an escape that ends the level. It's a relatively simple objective, but puts a new spin on the 'defend an area'-style objectives found in the original Alien Swarm campaign.


A broken floor looks down on hundreds of unhatched alien eggs

What Do Players Think?

Having delved into some of the reviews of Alien Swarm: Reactive Drop and sought out mentions of Research 7, I’ve been pleased to find people mentioning the ‘newness’ and ‘different feel’ of it, which was exactly what I was aiming for.


People cite the encounter with the Alien Queen as a unique experience, and love that it’s the only campaign set on Earth. While there’s much I’d do differently if I was creating it again today, I’m proud of this reaction and of what I accomplished as a one-man-team with limited assets.

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