Ballista are the tier 2 siege units. They are weak against all troop types and for this reason should not be used to attack armies on resource tiles.
Like all siege units they are strong against city defences i.e. wall traps. You should therefore always include siege units when attacking a player city with lots of traps. The downside is they are the most expensive tier 2 troop to build.
Ballista require roughly double the resources and take twice as long as to train than Battering Rams.
|1||200,000||320,000||200,000||150,000||200,000||4d||Academy 8, Skirmish Archers||Unlocks Ballistas|