It’s really a crime that in an age of Angry Birds everything, Crossbows and Catapults remains out of print.
The main thing I remember about the “rules” was that you were more or less told “make up your own rules or don’t, you have Crossbows and Catapults, why aren’t you making walls and knocking them down RIGHT NOW!?”
Crossbows & Catapults came with two sets of building blocks, “keep” towers, plastic miniatures, hard tokens, Crossbows and Catapults. You’d build your little forts opposite one another and take turns trying to knock them down. Or move your little men across the battlefield to tactical advantage. But mostly knock stuff down.