Don't let your enemy determine where you fight. Ignore the enemies in the tower, leave a few players behind in stealth for when they come out, and move on to your next objective. They can't defend their keeps and resources if they're camping in your tower.

The problem is that when half your forces have tunnel vision for the tower, there's no reasoning with them and your best bet is to just put your healers near the door and rush in all at once with rapid maneuvers, barrier, dodge roll, and all that good stuff.