Predicting the answers "use cross guarded tanks", no it doesn't work because the current aggro system is a bit... unfinished so lord just rampages around oneshotting anyone he hits while healers trying to res faster than he's killing. Add even a small def and any siege is doomed to fail.
There's also champion keep guards who aren't that strong as their master but still have shittons of damage and are a real pain in the ass, especially when trying to break a tank wall on the first floor.
To add, if my memory aren't failing me, on live, champion keep guards would appear after claiming keep by a guild and upgrading the claim. Such keep would become almost untakable for a regular pug. Even a guild warband would have some troubles taking it. And those champs were much weaker then we have here...
Suggestion? Make the keep guards 11-13 lvl and replace the champions with regular mobs. Because RoR mobs are much stronger than on live, even a 13 keep lord would be a serious threat. Who disagrees, try to kill a hero for tracker's or stalker's set solo

EDIT: wrong category, moved to suggestions