
Posting Hedon so you are aware this game is VERY Thief-like in atmosphere
Despite this game being mostly a GZDoom shooter, the atmosphere, level design and progression logic is very Thief-like. Huge non-linear levels, tons of secret passages, the sounds. It even has some references to the Hammerites. Average level takes around 40 mins.
P.S. As a bonus, some of the music is written by the mighty Alexander Brandon!