Wow, this was really really sick! You totally nailed the retro shooter gamefeel. I found myself relying on the shotgun for most of the game, but I still enjoyed the other weapons.
I really liked the use of lighting to differentiate the rooms. If I felt lost, I just had to remember what color lights were on the walls, and sort of 'follow the light' if that makes sense. It also made it easier to realize when I was going backwards (i.e "ive seen this blue room before, I'm going backwards")
Not sure if I was doing something wrong with the final boss, but it felt like it took me hundreds of shots to kill it. I ran out of ammo on all my guns and ran around picking up ammo and emptying clip after clip into the beasts head, but eventually killed it with a stomach shot (perhaps I was supposed to shoot it in the stomach the whole time?). Ended with 12:21 as my final time!
Great work!