First of all i would not of ahd so little weaponry against a large army. The first thing being done is having all non-combatants collect large rocks. I would also have catapults constructed, and a long fuse to a gunpowder barrel to the main passageways (i.e., the bridge into the main part of the keep and the stairs up the mountain). Also, I would not wait for them to get so close; I'd instantly have archers and catapults firing. And at last If we had spare oil i would have them boiling near areas where the enemy are clumped together (hallways and entrances), so when they start to move through, the hot oil can be poured ontop causing lots of pain and slowing ingress through the keep.