The largest and most magnificent covered cistern in the city is entered through a small building to the west of the Hagia Sophia Square. The ceiling of this forest of columns is made of brick and is cross-vaulted. It was built during the reign of Justinian I (527-565) to supply water to the palaces in the vicinity. Its 336 columns are arranged in 12 rows of 28 each. The cistern measures 140 by 70 meters. The columns are topped with capitals, some of them plain, but most in Corinthian style. The water level in the cistern changed from season to season. The water was distributed through the pipes at different levels in the eastern wall.