[TheDopp]

what is the gate for, if not to keep out people that haven't capped A and B?

To protect the king and queen.
A better question is what balance-breaking thing demo's can do on the battlements, in the walls and in the courtyard (when RED gets out to each of those first on round start)?
Worst possible case 14 demo's fly over the wall and somehow manage to camp both spawn exits after somehow killing everyone (who were "Prancin' aboot with [their] heads full of eyeballs!") while two folks (scouts for the sake of argument to speed things up) cap the two points that no one from RED bothered to jump down to before BLU's gates opened, and those same demo's keep spawn camping and not dying while the two scoots capture C. Of course this scenario isn't reliant on the demo's flying over the walls as they can split evenly and take A&B, then rush into the castle and camp it the same way, since there's no advantage to jumping the walls and encountering all of RED versus taking A&B and only encountering those that jump down off the walls, then killing the rest when the gate opens.

tl;dr - The demo is a fucking Highlander with magical powers, one of which includes running and flying over rocks like a boss, and it doesn't break the map.

Edit: Also, one more.