Investigate The Balvonier Tunnel System

In the Abbot's yard, hidden by a tent, was a cave entrance that led to a series of tunnels. Four stone archways marked with letters atop the keystones: S, W, B, T, presumably leading to the four main towns: Spindledown, Weatherby, Balvonier, and Thornyside. Will these be simply tunnels, or will these be portals? How deep do these run? Is this where the Abbots went?
Not only those, but the tunnels recently found underneath the Red Pony Tavern are also labrynthine.