Zamora was known as the ‘la bien cercada’, or well-fortified city. This was due to the three walls surrounding the town that made it an important strategic point on the banks of the Duero. The first of these is the most important. It was built during the reign of Fernando I in the 11th century, on the site of a former Arab fortress. It encloses the atoll where the Zamora’s old quarter is located. There are various access gates. The most noteworthy being that of Olivares, which leads into the Cathedral, and the Episcopal Palace. On the opposite side is the North or Doña Urraca entrance. The third gate is the Portillo de la Lealtad (formerly the Portillo de la Traición, or Treason Gate) through which Bellido Dolfos passed, pursued by El Cid Campeador after the assassination of King Sancho II.