Siege mode: the trebuchets do not work, only the onagers.
I should explain with a full description, but the basic idea is that this is the lull between assaults. You raid the other side, try to burn their catapults/siege stores, and then get back to your own lines. If you get near the enemy, they will wake up.
Attackers can use the ladders to get in and the sally doors to get out. Defenders use the sally doors to go both ways. If you hit backspace, you get a schematic of the battlefield showing your objectives and the enemy's objectives, but I'll replace that with icons marking the objectives.
If you burn an objective, you will not see flames. I need to add that, and I'll also probably make the catapult missiles burn. It's not altogether historical but it looks cooler.