I was looking at screenshots from the new mod 'With Fire and Sword' and it looks like they have an actual caravan on the battlefield when a caravan is attacked. There is a circle of wagons with the defenders inside and just a small gap to defend. Not sure how difficult it would be to add something like that to POP, but I think it would be interesting. 'With Fire and Sword' isn't out yet, so I don't know the details, but I have always wanted to see a caravan on the field when I attacked one.