Nexus 1.0

DOOM is one of the oldest FPSes in existence, and its engine is famously so robust that it can run on pretty much anything and also create pretty much anything.  From FPSes to RPGs to companions to a city living game, it has a ton of different mods and maps that can all do different things. One of many user-created maps is Nexus.

To install Nexus, you need to have ZDoom and unzip the WAD files into the GZDoom folder, which is pretty par for the course for DOOM mods.

These maps don't try to have a story. Rather, it's simply focused on gameplay, and it shows in the map design. Each section is separated by "airlocks", and it's very consistent - each airlock has resupplies, and since the author knows that you will be at full strength every encounter, the can design it to push you to the brink.

There are a few maps here - Prelude, Sideswipe, Leap of Faith, Hall of Heroes, and Redemption. Each of these levels has a different gimmick. For instance, the last level is a constant boss shooter, while the first  level is a simple room that dumps you into the thick of the fight with some zombies. Sideswipe has a J-shaped techbase corridor and gives a different flavor to all of the encounters throughout.

Overall, Nexus is a great series of maps that will test your flexibility to different encounters. If any of that appeals to you, then you should definitely give these maps a download.