20 tracks? You madman!
The sound design overall is perfect for that old school FPS vibe you were going for. Those industrial beats with strings and crunchy, distorted synths mesh really well. I loved all the bubble sounds in Caves and Sewers. Especially Caves, that sounded like something thick and viscous was bubbling, like mud or lava. Train is another track that stood out to me, I liked the use of the steam engine hisses as a rhythmic element and using the banjo to give it an old school railroad feel but still surrounded by industrial sounds. I also like how Cat and Mouse turns into a full techno track
The loudness of these tracks were a little uneven. I found that I had to keep adjusting my volume lower as I got through the OST. For example, Patient Zero is quite a bit louder than the first boss song Underwater Serpent. While variation in dynamics throughout a song is good, and atmospheric tracks make sense to be a little quiter than boss tracks, they should have a consistent max volume so the listener doesn't have to adjust volume settings. All of the boss songs should be at the same max volume level.
The textures and sound design were great and made for a cohesive soundtrack where I could imagine each level as I listened, and the amount you were able to make is quite impressive. Great work!