This is where the trebuchet came from:

This question comes up so many times in the message board I thought I would just clear it up here.

They didn't have it hiding somewhere. It was assembled on the spot in the yard during the battle. You see them take apart a bench press and use it's frame as the fulcrum for the trebuchet. Then they connected a steel drum filled with rocks from the wall as a counter weight.

The whole thing was placed on top of a steel rolling cart they rolled outside.

Yes it was a little over the top but still, just plausible enough, that it COULD happen.

"Common sense is not so common"