The oil industry changed Baku. Its population in 1807 was only 3,000; by 1900 it had increased to 250,000. Migrant workers from various parts of Russia, Iran and other places flooded the town; the local Azeris now were a definite minority. Before the advent of oil, the residents of Baku lived in a walled mediaeval town. The walls protected the city as a fortress. Some 500 meters of the walls still remain around that town which is now called Ichari Shahar (Old Town). They are 8 to 10 meters high and 3 to 4 meters wide. There were several gates. Some of these entrances, as well as five of the original 25 smaller towers and one of the 5 big towers, at intervals on the walls, have survived. “Inscriptions on these walls indicate that they were built by the Shirvanshah ruler of Baku in the middle of the 12th Century,” our guide said.