I had mostly avoided den defense until I killed the lead janniserry tonight.
All of a sudden everybody was attacking me and I couldnt get away... Ended up with 3 dens under attack. I finally won one though after 3 defeats.
BTW, anyone who likes the AC series should really check out one or both of the Batman games. As far as gameplay goes, they're actually very, very similar to AC. Same big open world, same emphasis on stealth, and the fighting mechanic is much simpler, but may actually be even more satisfying.