Good style and construction, consistent and well pulled off for the most part. Pipes! The layout seems nice, too.
Gameplay was fun on skill 2 with two or three slightly hairy situations for me - the five secrets I found helped get through without problems. I thought the crampedness was a refreshing change, didn't get annoying as one would expect.
Shame about the obviously repeating texture on the pipes. And damn that lift towards the GK door - the button would have needed a delay badly! The windows were a nice touch visually. The implementation is dodgy, though. Skiptextured func_walls around the water volumes or a skip/water combo like in apsp2 (btw similar vibe to your map) or even detail brushes, they would've been solid and not do the splash sound if grenades pass through.
If your map needs any special settings or the like, it should always be mentioned in the readme and news thread!
GG, looking forward to seeing more levels from you!
Demo