It´s a Mappack. UFO:Enemy Unknown (europe name) MapModules baesed on the classic "X-Com Ufo Defends" for Freedoom Phase 2, i called it "UFO-UE".
You need the GZDoomBuilder. Optimal function with GZD.Builder R2787. All Structure Contents (and some objects) remade from UFO EU into the Doom Universum.
When you use R2787 can some sector problems by copy the Modules.
GZDoom Builder 1.14 (r1685) works perfect.
This is not only beautiful, but plays well.I enjoyed it very much. :)