Why it looks like this

The old city, called Sur ('walls'), is exactly what its name says: a dense knot of lanes locked inside a ring of black basalt fortifications on a bluff over the Tigris River. The walls, largely from the Roman era and repaired by every ruler since, run about five and a half kilometres with dozens of towers.

Two straight streets crossing at the centre — the bones of the Roman plan — still connect the four main gates, while everything between them is a maze of courtyard houses. The modern city of wide avenues and apartment blocks spreads west, outside the ring.