I think you're correct, that the OP is using NickAltman et al's Siege Ladder Project. Created for the Lords and Realms mod, it's available as a standalone replacement for the external (walls battle) scenes for any mod that uses the native scenes (which PoP does). Every center is given at least two ways onto the walls: 2 ladders, a siege tower and a ladder, or 3 ladders for some towns, along with the AI meshes to make use of them in both attack and defense.
I've used this with PoP, had no troubles with it, and recommend it highly. Especially when trying to take a Sarleon castle with a siege tower, it's incredibly nice for the guys who aren't pushing to charge up the 2ndary ladder instead of standing around being pincushioned.
As always, other mods in combination with PoP are not supported by the PoP team. I can't think what this might break as it's just a scene swap, and I use it myself, but I've found some truly dumbfounding errors caused by tweaking things you'd never expect to affect anything else, so use at own risk.