The player operates a trebuchet to, well, crush the castle.
Castles are build from rudimentary big blocks. In the castle are a few targets: the king, some soldiers etc. Using game physics the player tries to collapse the castle, killing its inhabitants. In higher levels, castles become more complicated and more solidly build, while the player's choice of ammunition grows.
The player is guided through the levels in a static campaign, with short stories connecting one level with the others. The player is mostly also given some limited choices about what target (what level) to take on next.