Complex sentence: Yoshitomo's men were then obliged to attack the Rokuhara mansion, but failed in their assault, and fled Kyoto, meeting resistance along the way from the warrior monks of Mount Hiei who they had attacked in decades past.
Split into simpler sentences: Yoshitomo's men were then forced to attack the Rokuhara mansion, but ultimate failed. In the wake of their failure, they fled Kyoto, meeting resistance along the way from the warrior monks of Mount Hiei whom they had attacked in decades past.