Long story short: I tried to solo-siege a keep. I successfully got a part of the outer down below 50% (42%, actually), but it didn't flag; it didn't cut off the transitus links. What other criteria are there for flagging a keep?
Short story long: Opposing faction was busy trying to take a scroll and had emptied out this particular keep (Chalman/Thornblade), except for a few players. I wanted to try and cut off their reinforcements so I set up my ballista at the NW corner of the keep and started hammering away. Whenever I saw an enemy, I'd take the ballista down, stealth unil they passed, and pick up where I left off.
Yes, I was hitting the corner of the keep and not a side wall or door. Others have told me that taking down the corner does no good; when it collapses, debris blocks the path so you can't enter the keep. That's fine... Getting inside wasn't my primary goal anyway. But then what's the purpose of allowing significant damage to part of the keep wall when it has no effect on the overall health of the keep?