Tunnels of Andor

"The ransom was supposed to be delivered to the Vale of Denaria. If they're holding Gwen anywhere, it has to be there. Which means, it'd save a day's riding if we cut through the Tunnels of Andor."

- Arthur to Merlin

The Tunnels of Andor are tunnels located near the Camelot/Mercian border. They run through an unnamed mountain range and are infested with Wilddeoren.

History
While journeying to Mercia to rescue Guinevere from bandits, Arthur and Merlin cut a day off their travel time by taking a shortcut through the Tunnels of Andor (Lancelot and Guinevere).

Arthur and the Knights of the Round Table also used the Tunnels as a shortcut while traveling to the Isle of the Blessed. According to Arthur, using the tunnels would take days off their journey (The Darkest Hour).