There are some spells and special abilities that to damage that can not be defended against. I think one of them is boulder-throw or something like that. I'm also pretty sure the drake you can get from the dragon egg random quest has a special ability that ignores defense. So yeah, other than that you'd be invincible. You'll still get one-shot if you rely on it though and don't have a few troops backing you up. Still, that does need to be fixed since I think I've had issues where enemies had negative defenses and were unhittable. I don't remember what it was that modified their defenses down since I was too preoccupied with rage-quitting that short lived sandbox game.
