The walls of Diyarbakir, built of the dark basalt so characteristic of the area, are 12m/39ft high, up to 5m/16ft thick, and 5.5km/3.5mi long. All but six of the original 78 towers still survive, and four gates, the Harput Kapisi (north), Urfa Kapisi (west), Mardin Kapisi (south) and Yeni Kapi (east). Two of the three arches of the Urfa Kapisi were bricked up in 1183 by the Ortokid ruler Mohammed. Gazi Caddesi, running north through the city to the Harput Kapisi, is the old cardo maximus of the Roman town constructed in 363. The Harput Kapisi (or Bab el-Armen, "the Armenian Gate") has a niche on the inner side decorated with reliefs of bulls, birds and lions (probably 10th century). The gate as a whole is an amalgam of Roman, Arab and Byzantine elements. On the southwest side of the city, the massive Ulu Bardan bastion, 24m/78ft in diameter, built to reinforce a westward-projecting section of the walls, is Byzantine in origin. Further strengthened in 1208 it comprises two casemates, one above the other. At the extreme southwest corner of the wall is another equally massive circular tower and, further east, the Nur Burc tower embellished with reliefs and a Seljuk inscription.Jutting out abruptly from the south wall beside the Mardin Kapisi is the Kici Burc bastion (1029-37) which has five vaulted rooms.