Defend the castle walls! Place siege weapons and archers.
Catapult: Click-drag from tower to aim arc shot. Ballista: straight penetrating shot. Boiling Oil: click near wall to pour. Trebuchet: long-range AoE.
Enemy siege: Battering Ram (destroys wall), Ladder Crew (climbs over), Sapper (tunnels, invisible until close), Enemy Trebuchet (damages your towers).
Click wall segments with Repair selected to fix damage. Click towers to upgrade.