The Hospitaller Knights gate.
It was built by the Hospitaller Knights as a gateway to the walled city in the 13th century.
It has a rectangular floor plan which finishes in a semi-circle. The entrance, with a horseshoe arch, is flanked by two towers. At the centre of the blind arches there are remains of an early Christian sarcophagus from the 4th century. It features a relief with the emblem of the cathedral that represents the imposition of the chasuble to San Idelfonso under the Sun and the Moon, hence the name of the door.