Making each Arrow tower worth 1 Victory banner and pushing the Banner count up to 6 would be an interesting variant for this scenario. The Minamoto Victory Special Rule "The Minamoto army may not win unless it has a unit on or over the Taira side of the hex line of the Palisade at the same time as it has 5 banners." could be removed.
“I dreamt of worldly success once.”
― Miyamoto Musashi