Catapults, as far as i know, only require ordinary "stones" you find all over the place. Are you sure you didn't built a ballista by mistake? If you build a fortification, or dig one out, a catapult should be able to fire though it fine. It does not have to be high up, though i don't know what the effects of having higher altitude catapults would be, so it might be worth trying out.
Catapults will fire regardless of targets or not, since it requires only a "fire" command. The problem is that it's up to the operator to find a target for it, and on top of that, he has to have enough skill for it to hit something, which can be very difficult if hes not skilled in siege operating. What alot of people do is they let their siege operators fire the catapult alot for training, since it only needs stone for ammo, then they put them on ballistas (ballistaes?) when the enemy sieges come, because ballistas are more powerful and i believe more accurate. Also, siege operators are civilians by technicality, so remember; if they see goblins getting to close to them, they will abandon the catapult/ballista, even if they are behind a fortification.
I don't know the range on catapults, but i know that ballistas fire very very far away and can kill in single shots.