just wait for them at the next side of the bridge.

the crazy AI catapult units will kamikaze across the bridge, and pushing their siege machines with them, and wait for your men to slaughter them when they are still pushing the machine.

my units of 4 peasants (originally intended as cannon-fodder for my general-leveling activities) blocked the bridge warm their hands nightly on the corpses of the catapult crews, the frame of the catapults and balistas, archers and some hapless militia that the crazy AI push across. nuts.

trebuchets has longer range, so you may have to bait the AI into coming across by pulling back your units.