The medina's amazingly well-preserved walls measure about 33 feet high and 7 feet thick, and are 15 km (9 miles) in circumference. The walls are fashioned from local reddish clay laid in huge blocks. The holes that are visible on the exterior surface are typical of this style of construction, marking where wooden scaffold supports have been inserted as each level is added. Until the early 20th century, before the French protectorate, the gates were closed at night to prevent anyone who didn't live in Marrakesh from entering. Eight of the 14 original babs (arched entry gates) leading in and out of the medina are still in use. Bab Agnaou, in the Kasbah, is the loveliest and best preserved of the arches. The best time to visit the walls is just before sunset, when the swallows that nest in the ramparts' holes come out to take their evening meal. A leisurely calèche drive around the perimeter takes about an hour.