In regards to tunnelling.
Usually the reason for this is to destabilize the walls rather than infiltrate... I mean imagine sending a ragtag group against a well establish enemy in their own turf inside a choke point... Nope... but what they do is built the tunnel underneath the wall foundations, support it with work... then burn the wood supports, causing parts of the wall to crumble. Now.. if its so simple why haven't so many castles fallen to this tactic? Well, its because 60-90% of all the castles out there are built on solid bedrock or have foundations going several feet deep. The rest either have moats/sandy/wet surroundings or even have multiple layers of wall fortifications. All in all sapping somehow imbalance the game or in it self rendered moot and unnecessary.
And since moats were a main defense over sapping (rather than an attack)... they would also be quite unnecessary.
My opinion would be to just build yourself ridiculously sexy defense rings of walls and mount on them over powered defenders.
And to take them down... pop out some trebuchets/catapults weaken large sections then take them all down at the same time and bust into that large hole in the wall.