I was playing custom battle on 1.5.6. it was on Simira Castle, I was playing defense and I noticed that the units could not go up the steps. When I tried I could not do it either. I have noticed things like this in any bandit camp that has stairs whether they are wood or stone in previous patches of the game. Something is wrong with the steps and the siege ladders. I don't think its an A.I. problem its a mesh problem. My suggestion is for the 3D artist to make all the steps shorter maybe even tinier. I think the same thing applies to seige ladders and towers the 3d mesh needs to be changed so that their are no physical obstructions. The NPCs just bump into things not because of their programming but because of the 3d objects not having the right size and angles allowing the NPCs and the character to climb stairs and ladders. So all of the stairs, ladder rungs, steeps and any other inclines need to be audited.
