After more 15v15 playtests these past few days, my initial b2 feedback seems to hold up, so please refer back to that. Additional comments:
Stage 1
This stage is fairly well-balanced. There are a few things I want to note:
This path dead-ends and is a hassle for less mobile classes. Perhaps add a way out?
RED could use a slight boost in defensive advantages in this area of the map. This could possibly be achieved by sealing off the BLU entrance to the pictured platform and relocating it to one of the two paths shown. (Also expand the rear of the platform a foot or two).
Stage 2
The area just before the 1st capture point is a bit spammy. Once BLU breaks through any initial defense right at the start of the Stage, they're virtually guaranteed to get into the 1st point building. The 2nd point offers RED few places to go once BLU reached the edge of the 1st point building closest to the RED spawn. The 2nd point sentry platform is very vulnerable right now and difficult to defend from. Some suggestions:
Restore the height/orientation of the rock shown to the same way it looked in b1. Remove the one-way gate. By doing so, you can dilute the mass of RED in this area of the map and help spread out the fighting and reduce the spammyness of this area. I'm not sure how effective removing the grate will be, but I'm interested in seeing how it plays.
Move the one-way gate to the indicated position. This allows RED to better defend from the sentry platform and dilutes RED further, while still allowing BLU an alternate route to the 2nd capture point area.
Seal off this area with wood or a grate (preferred). RED demos have a very strong and overpowering advantage from their ability to control this area from a safe position above.
Remove the indicated column to increase mobility and reduce spammyness in this area.
Expand this area to make it a viable sentry platform and move the health/ammo backwards OR extend the platform forwards.
Stage 3
I haven't observed this enough yet. It definitely needs improvement, though.
A final comment - please don't rush this to RC status. As the person who strongly advocated steel and put Fishbus into contact with Valve, I'm looking for a new map to push. Steel went through 18 revisions and extensive playtesting over a 6-month period before being ready for prime time. I think that's an example worth following.