If you get into the catapult (the very first siege weapon you can get), you can drive it up the walls on the towers on the main base of wintergrasp. The places where the cannons are 2 stories up, you drive around the outside moving towards the tower. you will start to drive up the wall, and then be able to dismiss and be inside the fort.
i have done this several times and it works every time.
someone confirm me please... its relatively easy to get off and very useful for killing cannons and just generally injuring them from the inside out.
if not, ill post screenshots next time it starts.