Last nite in WAR some order folks tried to take over a keep that we happened to be near, so we joined the twenty-dozen people who were defending it, as the battle moved from the plains outside, to the keep walls, to the keep interior, to... no, wait, it kind of just stayed there forever and ever.
Until Trask and Rowyn got bored and went to bed.
Defender advantage: The ordinary stuff -- chokepoints, terrain.
Attacker advantage: Ability to respawn and run back to the fight. Defenders have to get rezzed and that can take some doing. Especially if they let themselves get knocked into the middle of the enemy swarm.
But eventually (HOURS), the keep held -- the army of Order was pushed back out the door and routed across the fields of Dock Garage. Then, as everyone was celebrating their victory, we got a message over world-com that they'd actually just all gone to a different keep and were invading it instead.
I think the main lesson is that defending a keep is really boring for melee folks. Also, Trask is very good at hiding from healing spells -- basically, his 'I need to break LOS' and my 'oh, did I forget to heal Maledictus?' triggers were both at about 50% health, so I'd get a line-of-sight error every single time, and have to run up the ramp chasing him.