Contested WP
And a thief somewhere gains +1 ego
Stealth traps. Perfect for garrison watergate and get a buddy to put one down at hills. Over and over until the guy gets frustrated/killed.
You can only place one trap per map per hour, so work with a friend.
I think a more reasonable solution would be limit it to siege damage initiating contested status. A single person attacking the door is not a credible threat to the keep unless left to auto attack for a day or two. If they had to setup some form of siege at least they could be spotted and repelled. And I wouldnt have a problem with arrowcarts targeting the guards as counting as siege damage, just something that requires more than 1 person taking 1 swing at the door and vanishing into stealth.
I think it is dumb how a keep will become uncontested if the defense timer expires too. How is it a siege if people can just teleport in zerg style.