Even easier way to counteract saps is with a moat.

I remember an old, old (well mid 1990's) game set in medieval times where the POV was top down and the castles had moats. You had to send in peasants or other cheap units to fill the moats in so you could sap/use ladders. I'd like to see this back again and would be easier for the game mechanic I'm guessing (rather than Underground:TW).