"Burned by the former dynasty Emperor." Gu Jinli frowned, "It seems the Rong people have found the map left from Beicang Prefecture's construction during the former dynasty, or they've found the descendants of the Master Craftsman."
Qin San Lang nodded, agreeing with Gu Jinli's statement. "The only question is how far they've excavated now? Digging a tunnel is not a simple task; the ground has to be deep enough, and the tunnel long enough."
The defenses of the city walls include both above and below ground. Underground defenses include mechanisms and many barriers, with various traps buried beneath the moat surrounding the city. If the tunnel isn't deep enough and hits a trap, the water of the moat will suddenly drop, and the garrison soldiers will know something's happened underground.
In short, digging a tunnel is a massive project. Even with the map for city construction, you need a large workforce for excavation and to deal with the dirt and stones dug out.
