Alien Swarm is an open-source, free-to-play, co-op based, top-down shooter, where you are tasked with surviving against hordes of alien enemies. With intuitive controls the game is easy to pick up and play, which is great for a multiplayer focused title.
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Pros
Pro Easy to pick up and play, even for beginners
The game consists of tried-and-true top-down shooter mechanics that will be familiar to most players; it is reminiscent of older arcade and console shooters.
Pro Anyone can download and play the game as it is free-to-play and open-source
The game is completely free and has no microtransactions or ads.
The source code is also free and accessible, so the game can be easily modded.
Pro Easily play with up to 4-players in online co-op
Alien Swarm allows for up to 4-player online co-op, which makes for an easy way to play with a small group of friends or strangers.
Pro Convenient offline LAN support
Alien Swarm offers offline LAN (local area network) support, meaning that the game can be easily played locally with each machine connected together in the same place.
Cons
Con Can get a bit repetitive
The gameplay doesn't change much from mission to mission; you’re generally just shooting aliens over and over again.
Con Gameplay is too short
The main story only takes about three or four hours to complete, which means you could beat the game in one sitting pretty easily.