Higher ground along with few entry points to enter are always the best places to be. When you're on the edge of the keep shooting down always gain a target shoot then back away and gain another vantage point looking down. Simply just move around and don't stay still. Once you stay in once spot you're easy pickings.

The entry point up the stairs is a great choke point. And like Dalmmar said, once you see an enemies head bubble it. It should be a delayed bubble for the 2nd or 3rd person though. Let the one bubble grab them and the other person hold theirs for a little longer then bubble again. That will hold them in one spot longer and more vulnerable to nukes. Also from top of the keep you can jump to the cannon tower to the west, and gain some time to heal and such granted nobody is up there waiting for you.

You can jump to the top of the fighters guild as well to evade and heal and make your way to the mages guild. In which also is a great place for defense. Since the very top has a small spiral entry point to the top.