Some Q's about weps and effects

I'm wondering how the battle algorithms work.

I know ballistae kill people from outside the range of the towers, don't seem to take down as much in the way of fortifications though.

Battering rams - what are they best at taking out, what kills them?

Abatis and traps - what cleans those up besides busy little warrier men and cavalry inhuming themselves on them?

Can't my academy develop a decent flamethrower and/or small tactical nuclear device - what am I paying gold and goods to those boffins for? %)