Okay I havent played in a while but what the hell is it with thunderhead keep? Did they change something in the later patches? I've done this mission with 3 characters and completed it on the first try with 2 of them, 2nd try on the other.
Fastest was when I did it with my monk. We had a group of 4 warriors 3 monks (1 smiter, 1 protection hench and me) and some other class I cant remember.
Just take on 1 group at a time and call targets. When we got to the keep we just started chatting and fucking around. 1 warrior went tanking one side of the gate, the smiter monk went afk every 2 minutes and everyone else was fooling around at the other side of the gate. Only used the catapults once or twice, much more fun hacking them up close and personal

.
I dont see how you can fail defending the keep if not everyone is afk. The minions come at you with like 5 minutes inbetween (okay a little less, but still a long ass time) and they're in groups of like 3(?), which is not that hard.
So has this mission changed or what?