Guard NPCs attack passive monsters during sieges
It is annoying that they focus on slaying the evil croconut trees and bifs instead of focusing on the invading monsters. They should also ignore resident monsters such as maggots and crocs. If possible, they should use a whitelist so they only attack the aggressive monsters.
Griefing players can also plant oaks to deliberately distract guards, causing their deaths since they will not fight back when a stronger monster attacks them while they are fighting the obviously very evil oaks.