The year is 878 AD, vikings have once again tried to invade the lands that now form germany, and brought their boats up the Elbe river. But near Dresden, a small castle in the works atop of the mountains that flank the river there, repelled the invaders while being outmanned 4:1 and with little in terms of training or equipment. The only thing they had are catapults, which may sink their slow ships (as they are pulled against the current) but can do little against infantry. Which would soon take revenge for any boat sunk as they are too unprecise.

I have an idea of how, but I would love to see your approaches.