The gameplay is good, the layout is fantastic, even better than moonblood's, perfect usage of heights there.
Yet there's still things to suggest, replace the constant percussion in your MIDI to something more mellow and ambient (maybe heavy bells or a bass drum, I think that'd compliment the map's atmosphere).
One of the armours should be replaced with a trail of armor bonuses.
The hot red building should have a less abrupt texture scheme, something that will fit with the rest of the map, you may want to check out the textures at cc4-tex.wad if the stock textures aren't enough for you.
Make the lift to the rocket launcher more noticable, in my FDA I only found it while looking for the last secret, some candles or pickups could make it easier to comprehend that it's there.
It's possible to break the path to the red key and make it inaccessible in a couple ways, one of them can be done by lowering the lift when you didn't raise it yet via the walkover effect linedef, pushing the button that's sunken inside the building, falling off without activating the lift will lead you to a stuckage.
The other way to break it is by pushing the sunken button from below the building, then pushing the first button without activating the lift or vice versa.
Most of the flats in this map are the Doom Builder default ones, looks unfinished and out of place, the gray variant should be replaced as well, they just look too technological, that kind of selection belongs to 90's themed wads, especially in such a significant amount, but I'm pretty sure you just forgot.
There are a couple of misalignments near the exit door.
FDA:
http://uploads.doomshack.org/gnf04fda.lmp
