Passage: The Walls of Jerusalem (Hebrew: חומות ירושלים) surround the Old City of Jerusalem (approx. 1 km²). In 1535, when Jerusalem was part of the Ottoman Empire, Sultan Suleiman I ordered the ruined city walls to be rebuilt. The work took some four years, between 1537 and 1541.The length of the walls is 4,018 meters (2.4966 mi), their average height is 12 meters (39.37 feet) and the average thickness is 2.5 meters (8.2 feet). The walls contain 34 watchtowers and seven main gates open for traffic, with two minor gates reopened by archaeologists.

Question: How long did the Walls of Jerusalem take to build?
four years