Rise of the Twilight Empire
Bowels of Hammerfall
Old Hammerfast was built over a geothermal vent, which provided the fortress with heat. The tunnels underneath Hammerfall still lead down to those vents, and provide an escape route into the Dawnforge Mountains should the city ever face a siege it cannot withstand. Few venture down into the bowels with any regularity, however. The veil between worlds is thin down there, and monsters sometimes slip through.