I would really love to see moats added to city upgrades. And maybe even a unit specifically for tackling them. Like a team that chains rocks into the moat so you can get your ladders up. Or maybe a team that dig's, then rallies the dirt into the moat. Maybe put it on a timer... If it the team takes casualties, it slows the progress of the dirt/rock bridge. If it gets annihilated progress stops, and you might need to pull some of your infantry to get it completed.