The town earns its name from the nearby Castillo de San Martín, which was an improvement on the castle that the Romans built in the same location.
Descending from the castle overlook the camino arrives at a roundabout littered with old arrows and new arrows. At this roundabout, which is now cut off by guard rails, the camino does NOT turn right. Instead it follows the road a bit longer to the next roundabout. At this roundabout it does NOT turn down the N-632 to Muros de Nalon, but rather takes the second exit to pass the hotel in Soto del Barco.