A trebuchet can launch a 90kg stone 300m. 90kg stones are difficult to get. However....
•average weight of a Rocky Mountain goat is 93kg
•goats are much easier to get than huge rocks due to farmers
•goats are already raised for slaughter regularly, and getting slammed into a wall isn’t any more inhumane than current methods of execution used by butchers.
•goats are smaller than stones, but heavier, thus making them cause a larger impact upon collision
•for the same reason, goats are also easier to hook up to a trebuchet than stones
•goats are more aerodynamic than stones (see diagram below)
Conclusion: we should be sieging castles with goats launched from trebuchets