how about farms producing the food needed to keep your units in fighting condition?
If you are a tad more cynic, make it more realistic, and have farms on the map ready to be raided. You cannot raid a farm too often (else the farmer will just pack his stuff and leave), or maybe you can only raid it once (because medieval soldiers weren't known to be nice to farmers they raided, they often killed/raped the inhabitants of the farm).
Other than that, you can have castles/outposts as supply depots... besides guarding an important road or pass, and sometimes giving shelter in case of an attack, that is actually what most of them were built for. To support an army passing through with food and other supplies.