Oh wow, where to start...
Highly polished, well put together and fun to play.
Gives you the chance to approach problems in a variety of ways, going stealthy, going guns blazing or going on a totally different path.
Achievements, secret rooms and breakable windows, and breakable light and background physics objects?! I don't know where you found the time to put so much effort and polish into this, but I'm glad you did! Heck, two playthroughs and I still don't know what the passcode door is about!
Truly a gem, not only in the context of a metroidvania game, but in general.
That all being said, three minor gripes:
1. Jumping from ladders is tricky
2. Really wanted to have a laser sight on all weapons
3. Shotgun and Sniper have a huge delay on firing after switching to them
*spoiler*
The escape sequence had my heart pumping, and that's not the caffeine talking!