Sapping

If you can dig underneath a wall, you can collapse it by removing its foundation. In general it takes an hour of digging to weaken a section of wall to the point where it takes 2d6 ×10 points of damage. Using multiple diggers cuts the digging time: two sappers will deal 2d6 ×10 points of damage every 30 minutes of digging, and four sappers deal the damage after 15 minutes. If your sappers are unusually strong (Strength 20+), they dig twice as well, and any Medium-size or larger creature with a burrow speed digs five times as well. Note that move earth isn’t helpful here, since it cannot be used for tunneling and doesn’t affect structures.

Naturally, defenders won’t sit idly by while you undermine the walls of their stronghold. Given the time it takes to sap a wall section, many armies begin some distance away and tunnel underground until they reach the wall, rather than dig from an exposed position at the base of the wall. The stronghold’s defenders sometimes counter by digging their own tunnels outward from the stronghold. When two tunnels meet, the resulting skirmish can determine who wins the larger battle.