Add transparent bulletproof glass to sniping corners to reveal corner snipers.
Make the battlements terrain bumpy so that strafe-sniping is difficult.
Angle and add a few side boards to the bridge so spamming it is harder.
Widen the bridge to allow spies reasonable passage without bumping into people.
Add a few shadowy side crannies inside base for spies to hide in.
Add a few tiny ammo-boxes/health on the lower level for spies to recharge cloak.
Remove the stupid ramps at the main entrance to fort, just have level ground.
There should be no way for a single sentry to cover all entrances to second floor.
The water in the sewers should be shoulder-high all the way up to resupply corner.
The resupply room in the basement should be part of the respawn cycle.
And as a general rule for the whole map -- everything is too orthogonal, boxy and bland. I would like to see the designer of pl_cashworks take a scalpel and paintbrush to 2fort. It's a classic map, but it needs more nuances, nooks and crannies, rough spots, divits, odd angles, etc. And it is butt ugly. Spies and scouts are royally screwed, as well. The forts walls are too close for scouts to make a hasty retreat, don't have tiny health packs for scouts to grab for survival, have few spots to hide out for retreating spies. All-in-all, the map favors defense GREATLY. It should be nearly impossible to keep people out of your fort, hard to keep them off your second floor, but reasonable to defend the intel once they're in your basement. That would keep it at a near-frantic pace. I've set up sentries in the basement of an instant-respawn 2fort, walked away to use the bathroom and go get a snack, come back 10 minutes later to find myself still alive, with 5 kills by my sentry and the team scores still at 0-0. This is not how a TF2 game should play.
It would be neat if there were time-delay back entrances with secured gates on each fort. If your team is psychotic about defense, the alternative fort entrances (harder to defend than the standard ones) open up after 10 minutes of no one on the opposing team touching your intel. Once intel has been touched, timer resets and gates lock for a while.