On November 20 the Chinese Army and teams of conscripted laborers began to hurriedly bolster Nanking's defenses both inside and outside the city. Nanking itself was surrounded by formidable stone walls stretching almost fifty kilometers (31 miles) around the entire city. The walls, which had been constructed hundreds of years earlier during the Ming Dynasty, rose up to twenty meters (65 feet) in height, were nine meters (30 feet) thick, and had been studded with machine gun emplacements. By December 6 all the gates into the city had been closed and then barricaded with an additional layer of sandbags and concrete six meters (20 feet) thick.