Something I'd never seen before...

[Captain_Aramsham]

My Roman forces are sieging a Hayasdan town. There's a Sauromatae stack nearby (my allies) so I decide to assault. On the battle map I'm to the south of the town; the Sauros are to the north, with no siege equipment. I put 8 units of spearmen with battering rams on the west side, figuring I'd send them round to the north to bash open the gate for my allies. As the battle starts I order four of them to drop their rams and just run to the north side, the other four slowly push their rams around. I bash open the south gate, and my main force rushes through. I bash open the north gate, and my spearmen and the Sauro HA rush through. Then a third 'gate destroyed' cutscene comes up. Puzzled, I zoom my camera across the battle map. It turns out that the Sauros had sent a unit of foot archers round to pick up one of the battering rams I'd abandoned on the west side of the town, and used it to bash open the western gate.

tl;dr - I didn't realise that Allied forces can (and will) use your siege equipment.

[Thracian Princess]

Hm... that's an interesting turn of events! Can't say I've ever seen my allies do that. Usually what happens with me is my allies break a hole or two and then die in a choke point, leaving me to take the town by myself.

[Ferdiad]

Yep, the AI is only useful as a Meatshield.