A torsion engine of two skeins of twisted sinew driving a pair of stiff arms, spanned by a windlass and loosing a yard-long bolt. Light enough to be carried up a stair and mounted on a wall-top or a tower platform, and worked by two. Against a formation it punches through a file of men; against a gate it does nothing worth the name. Its virtue is that it is aimed rather than lobbed, so a crew that can see a target can hit it.
Two crew work it: one to wind the windlass and lay the stock, one to load. Spanning and loading is the better part of a minute between shots.