"like weeks or even months of effort, during which time the enemy might be able to bring in a relief army or ally. In RTW the sap takes no longer than moving a tower up and unloading your men, or knocking down walls with artillery. You're supplied with magic mole men, as someone else said."

Sapping in RTW takes months. You have to wait a turn just to build the sapping points. Sure once you start the battle it takes only a minute to bring down the walls but in terms of results the game does not give you an unrealistic advantage- you wait one turn and then you breach the wall. To me it makes no difference whether the wall is already breached when the battle starts or whether it is breached during the battle at some ridiculous rate. Same difference.