Originally posted by Polonius Ulf:

Wouldn't you also like them to hit the seeker soldiers? The fliers aren't as dangerous, it seems to me. A tactic I want to try is to get a seeker soldier facing me with its rear toward a ballista. Of course, if the bolt misses, the player is also in the line of fire. But that's my idea of fun.

Sure, if you're in mistlands. In fact, ballista missiles vs. seeker soldier's bouncy backside is probably the best value you can get out of them. But soldiers won't spawn outside of mistlands.

So, as always, it really depends on circumstances how much use you will find for them.

Meadows base: Seekers spawning at night
Forest: Trolls, obviously, and nightly seekers. Wooden missiles should suffice.
Swamp: Maybe wraiths. Wooden missiles.
Mountains: Drakes, with wood.
Plains: This is where things get a bit tricky. I've never had a base of my own in plains so I can't really comment on that... High-level nightly fulings are probably the worst threat in the biome. You'll need black metal missile to stagger 1* fulings, but 2* cannot be staggered by ballista for now.
Mistlands: Wooden missiles for hares, black metal for the hostile ones. And maybe dvergr if you're particularly cranky neighbor.