Setting up a Siege Weapon should invoke a 5 or 10 second "timer." This leaves a person vulnerable while setting up siege weapons, and helps reduce the brainless tactic of oil pots everywhere.
Currently, people can just stealth up in a group, drop 3-4 oil pots and anyone who has seen it enough times knows its not good for the game.
The best solution is to impose a timer.
Within; Without.