Hey Grizzly, I haven't played in a few versions but I was excited to read about a21 so I've done a little run-around. Here are my thoughts in the order I notice them.
GENERAL
The triggers for your spawn doors aren't very long. I run into them opening as any normal speed class. Just lengthen them maybe 64hu.
http://puu.sh/iCIrb/146838ea3f.jpg
Crouch jumping onto this barrel causes me to get stuck underneath this dev-textured railing here. Just flush it with the wall or remove the barrel and use the standard 2-crate method so I don't have to crouch jump.
http://puu.sh/iCIFY/03df11582e.jpg
Unsure why you didn't block off this area. You have health and ammo right next to it, why not throw them in there and give that little space some meaning!
http://puu.sh/iCINR/2c53f5e4cb.jpg
Obvious nodraw behind these crates.
http://puu.sh/iCIW6/1a3f5936bb.jpg
I assume you made this as deep as it is to allow people to extinguish themselves? Feels odd to walk through and be submerged for a half step then suddenly be completely out of the water. But I don't know how else you could do it if you want people to be able to extinguish themselves, short of widening the water. Also, the middle support structure in this screenshot is off-center of the actual bridge. Looks like it needs to be moved to the right maybe 16hu.
http://puu.sh/iCIZY/365c633586.jpg
Doesn't look like you're using areaportals in any of your doors as you're rendering a lot of stuff areaportals would fix. They are extremely useful and don't take much time to setup. You could even add some
areaportalwindow's to the windows overlooking the point and the semicircular computer room windows with a black textured buddy entity. Alpha is always a good time to start optimizing.
http://puu.sh/iCLCQ/724eda52cb.jpg
http://puu.sh/iCLBT/9e8833048d.jpg
http://puu.sh/iCM39/6e599e4f60.jpg
MINOR
Even in alpha, I'd really like some ambient noise. Just sprinkle a few env_soundscape's around using Gorge's outdoor soundscape with the radius at -1. Should take five minutes. Going off on a tangent here, I don't know if you've done soundscapes before but once you get to the point where you're polishing things and refining your soundscapes, I'd highly recommend using
triggerable soundscapes. Way easier to control.
Not a big deal at this stage but dynamic shadows have a tendency to render on the opposite face of brushes they're supposed to render on. Just disable shadows on your resupply locker props. They won't be missed.
http://puu.sh/iCKcT/257dcdb900.jpg
A small clip brush here would be nice.
http://puu.sh/iCKj5/513d4ee002.jpg
Your displacements don't reach the ramp to spawn on either side.
http://puu.sh/iCKkX/bc0b873d96.jpg
http://puu.sh/iCKn6/e4e5927386.jpg
Nor do they reach this wall, but only on Blu side.
http://puu.sh/iCKn6/e4e5927386.jpg
All the barrels on the point are slightly floating.
http://puu.sh/iCM07/067afebeb7.jpg
I'm itching to play a21, I hope you toss it up on a gameday soon!