Hero engineers.

I always get the siege workshop from the Dwarves, and upgrade the ballista to 3 shots (probably a typo, referring to the siege attack skill from version 2.1). The drop rate for the ballista is low, but I usually don't have any problems with it. I don't see any point in having a strong Dwarf unit without a ballista. At the start, on the second day, I build the forge and immediately buy everything. This won't have a significant impact on the city's economy if you play strategically later on. Then, if I get the chance, I take the tent, but I've never taken the catapult; I manage perfectly well without it, as Chaos is sufficient.