Put a metal foundation and on one side 6 metal doorframes high, extend that side of the foundation with 2 thatch ones en the 3rd again a metal one with 6 metal doorframes high so you can aim your cannon but the doorframes block the incoming boulders, for safety put metal pilars 4 high on each side of the last foundation, make sure where u build your 'U' trap, that you have a clear shot, this safes you from a lot of metal runs for fixing your cannon