Although mechanically different from crossbows, Ballistas function quite similarly. Ballistas are siege machines that are most effectively deployed at long ranges against other units rather than city walls. Their javelins are much more accurate and aerodynamic than the boulder of a catapult. Several ballistas can provide excellent cover for catapults and rams attempting to breach a city's outer wall.
Because of their heavy wooden construction, they are slow to move from place to place. These machines are also more fragile than catapults and can be more easily destroyed.