Medieval; Total War

You could hide one unit behind the keep in your castle and most of the time the AI wouldn't find you there. Ended up with situations where an army of thousands of soldiers would storm your castle, lose heaps of units to your arrow towers, finally break through to the centre and then somehow fail to notice your 10 men hiding behind the keep. Finding no one, they would simply leave and let you keep the castle.

Sometimes I love terrible AI.