Great job capturing the feel of a Petersen open plan map! It really brought back memories of downtown, especially the part where I got lost for a while. A few comments:
+ The lift timing to the secret-exit lowering switch is *REALLY* tight. I would make it a bit easier (just raise the lift) - timing isn't a level mechanic, and few id secrets were that hard
+ I really liked all the key rooms, they felt distinct, especially the blue key, which felt appropriately weird and cut off from the rest of the world for a Petersen map.
+ It's a bit tough to find the shotgun at the start due to the curve of the building. I actually figured out the chaingun semi-secret first (very clever, by the way)
+ The spiral staircase by the yellow door is really great staging, the way the fights play out is just beautiful.
+ The dead-end with the invulnerability is a letdown fighting-wise. Because there's no loop around it's effectively safe, and a good place to duck into, but at the same time it can lead to a big cluster of monsters blocking the alley. Connecting the far end is an easy solution, but it also points to a lack of other breather spaces nearby.
+ The archvile ambush is a great idea, but the long distance and monster pathing hampers it. When I played it raised a few monsters, but not too many, and I sort of wandered into it after blasting the main ambush. I can see a few solutions, either
> put in a few more warp points near the bridge to draw the player in that direction. I'd keep the monster count the same, but shift some from the other warps so the archvile one becomes the major post-startup fight.
> put the archvile on the closer half of the map when you warp in.