"Sapping, well, it was a way to force entry into a fortified structure. You dig a mine or sap under defensive walls, typically shored up by wood. On collapsing the tunnel, for example by burning the props, the wall would collapse... a skilled team of sappers could collapse your defenses during a siege and end it quick. Sometimes it could work in reverse I guess... you're cooped up in a siege, but a Sapper could create a underground tunnel either to get supplies in... or even to spring some sabotage out there. I never studied it a LOT ? But I saw something about it on a history show once"