Any discovery of León begins with its unique city gate preserved as part of the ancient and medieval walls, originally erected on a quadrilateral plan between the 1C and 3C, and rebuilt between the 12C and the 15C, with 72 semi-circular towers and a further 8 towers flanking the gates. These were built using Roman siege architecture techniques; today, 36 remain, from the Los Ponces Tower behind Plaza Mayor to the San Isidoro Tower.