During its long history, Jerusalem has been destroyed at least twice, besieged 23 times, attacked 52 times, and captured and recaptured 44 times. The oldest part of the city was settled in the 4th millennium BCE. In 1538, walls were built around Jerusalem. Today those walls define the Old City, which has been traditionally divided into four quarters—known since the early 19th century as the Armenian, Christian, Jewish, and Muslim Quarters. The Old City became a World Heritage Site in 1981, and is on the List of World Heritage in Danger. Modern Jerusalem has grown far beyond the Old City's boundaries.