Agent Smith

I'm pleased we agree that fire arrows can cause rams to catch alight.

And that once alight the damage increases.

And that non-flaming arrows on their own won't damage a ram.

(but we have yet to work out exactly what triggers the catching fire, and whether being damaged by ballistas makes them more flamable.)

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As for cannon towers - well if you're lucky and the AI army has a couple of units of siege artillery that it uses in an attack, then your towers will fire a mixture of fire and solid shot at the artillery units. Luckily the rest of the AI army is usually nearby and that means rams and siege towers also get hit.

In one battle the Pope was "attacking" marseilles (IIRC) that I was holding as England. Settlement had towers that fired cannon balls and flaming shots. Pope sent forward 2 trebs and a couple of mortars to attack my walls - and they made a couple of holes in them before they were destroyed. But so were 5 rams and a siege tower. All rather pointless as the gates had been opened and they could have simply streamed in before I got my defence organised.

Interestingly once the siege artillery is destroyed the towers stop shooting as the rest of the army is officially out of range - even though they have been killing 50% of the army whilst "out of range". (The joys of over-shooting and bouncing cannon balls.)

Firing at a moving target cannon are pretty hopeless, unless the target is a slow moving mass of troops.