Amazing game! Great weapons and very distinct enemies (the sprite work is outstanding!), and solid level design throughout. At first, I thought the levels were going to be simple 2.5D layouts like in Wolfenstein 3D, but they got more complex as the game progresses - nice touch with some id easter eggs, like the first two levels of Quake.
Just a small remark: I think the very last level is too hard for my taste (at least the 2nd of those 3 bosses).