[Taqwus]

Castle damage is based on siege might of attackers versus defenders; size matters, if memory serves, as does mindlessness, flight, and the 'siege' token (some units have advantages besieging castles, some have advantages defending them). If the attacker has an advantage, the defense total goes down, and vice versa. Gate is breached once defense reaches 0. Magic can also be used to damage or strengthen castles.