Wow, that's an cool fort. It's kind of impressive to look at, allthough I have doubts as to how well it would actually work.
On an uncrowded server, your enemy would could get in while you respawn (ie build a ramp with engineers kit) and then he has a clear path to the ball with a quick exit.
On a crowded server, teammates would get shot up while they dash to the tower (and try to get in).
Nevertheless it's a cool fort. You should submit it to the screenshot contest. It's a pity we rarely get to fight in these masterpieces