Stage 1 I think is flawless, it has a great flow to it with plenty of alternate routes and no particularly upsetting areas.
As everyone else has mentioned, Stage 2 has it's problems. While I've always been a fan of the long passage after cap 1, the balcony is simply far too overpowered.
2 Engineers with the aid of a medic and a heavy can hold an entire team here, and while a skilled Demo with an Uber can clear it out, it takes far less time for the Engie crew to setup again than to build another Uber.
The mineshaft is a highly unused area that I think is in your best interest to consider as an additional alternate route.
*Apologies for the quick scribbles*
The addition of a path that connects to the mineshaft would give Blue a way of bypassing the balcony, without giving them too strong an advantage. The paths between the balcony and mineshaft are confined and easy for a heavier class to lock down, so you wouldn't be crippling Red. You'd also finally see some use out of that great mineshaft.
These are just simple sketches to illustrate the placement. You've got a lot of open space behind that mineshaft to work with, and the perfect distance for it to be a short walk.
The other area to consider was here, which would at least force Blue to cross Red's line of sight and risk getting sniped:
Although I quickly realized you have the side tunnel immediately behind the wall,
However you may feel it's worth reworking.
Stage 3 I feel is strong as well, although once Blue gets past cap 1 I feel Red's options are severely crippled, and in my experience they get rolled 80-90% of the time. Obviously you designed the final area to be open, similar to both of VALVe's final PL map areas, however Red is missing a few crucial Defensive positions for both sentries and other classes. Little nooks with a lot of wall so Demo's can't sticky Sentries on an angle, height advantages etc. Using the environment you've created, here's a rough idea that hopefully you find useful of how to rework the area:
A 2nd level to the right building, which quite frankly seems useless to me since it's windows are blocked off and the door and room is cramped. The upper level with the addition of a balcony and the catwalk to the left upper ledge gives Red defenders difficult targets and allows them to build a strong defense while supporting Sentries on the lower level.
If anything, open up that right building and give Engies a lot of breathing room.
Hopefully my scribbles give you some ideas on how to rework your map, which still remains my favorite custom map. Best of luck to you, I'm sure you'll come up with something great
