The best way to enter Frías is by the 143-metre bridge that crosses the Ebro. It is known locally as the Roman Bridge, but in fact, it was built in the Gothic style on nine arches made of stone. The engineering work dates from the time of Alfonso VIII , and has a small tower where mule handlers and merchants paid toll charges to cross.That part of the city on the riverside is protected by ancient walls with a castle at one end and the Church of San Vicente, which also had a defensive purpose. In the middle is the Medina gate, used by the Velasco family, the lords of the city, to enter. The cliffs of El Molinar form an impregnable defensive wall against attackers, facing the Obarenes mountains.