Retro FPS game Intrude now on Steam

Intrude is a first-person shooter inspired by old FPS titles like Wolfenstein 3D and Doom. There is an old underground base, abandoned long time ago, suddenly taken over by some kind of private military and emitting a strange signal. Your mission is to get in, find your way up through fifteen levels in one of three game modes and destroy the source of that signal. There is a lot of hidden data scattered across the base. Try to find as much data as you can to find out who they are and what they are up to.