Don't think this has been mentioned before, but horses can reverse up extremely steep inclines.
have a feeling its hardcoded but worth a shot
This is a issue with Warband in general as far as I know. However most maps should be fixed with barriers to stop this by now, if you find any exploitable places please inform us
got a feeling theres a place on the new mountain pass that might be exploitable, will check.
EDIT: scrap that nice placement of invisible barriers stops it
Reminds one of The Elder Scrolls, doesn't it
It's hardcoded. The only mod I know so far that's beaten it is cRPG, because of its implementation of WSE, which the devs of this mod are very reluctant to use, due to its similarities to spyware.