Sapping is something you do on the battle map, not the campaign map. Personally I don't bother with it that often, but it is useful, particularly when you want to get large numbers into a city quickly, or when the enemy has strong units on the walls (Sapping works very well against Romans, I have noticed) It's also good for cavalry-centric civilisations, as the breachs are good for getting cavalry inside.

The problem, of course, is first of all that you don't get to choose exactly where the sap points are going to be, which is a pain, and also that you don't capture the walls and towers, which can be a problem as towers and units on the walls can fire down onto your troops inside the city and cause heavy casualties.

Antagonist