From their website:
The Roman Siege Engines have started showing up at your local game store, and free Ballistas will be given away at Gen Con, so it’s a good time for another look at these innovative limited-edition figures.
Each Siege Engine box contains two four-peg figures and four base cards. Unlike the dragons and sphinxes, the base cards for the siege engines have slots for additional figures. This means that the ballistas can push. Because they are constructs, you can heal them with the Artificer special ability.
The unique special ability of the ballista is Direct Fire. This does guaranteed damage (like Fireburst) at a range of four formation bases. Direct Fire is a ranged combat order, not a special order, so it costs two order points, but you can push to do it twice in one turn. Each ballista can do a maximum of two points of Direct Fire damage per order.