title: Persepolis
context: Darius I built the greatest palace at Persepolis on the western side of platform. This palace was called the Apadana.[30] The King of Kings used it for official audiences. The work began in 518 BC, and his son, Xerxes I, completed it 30 years later. The palace had a grand hall in the shape of a square, each side 60 metres (200ft) long with seventy-two columns, thirteen of which still stand on the enormous platform. Each column is 19 metres (62ft) high with a square Taurus (bull) and plinth. The columns carried the weight of the vast and heavy ceiling. The tops of the columns were made from animal sculptures such as two-headed  lions, eagles, human beings and cows (cows were symbols of fertility and abundance in ancient Iran). The columns were joined to each other with the help of oak and cedar beams, which were brought from Lebanon. The walls were covered with a layer of mud and stucco to a depth of 5cm, which was used for bonding, and then covered with the greenish stucco which is found throughout the palaces.
question: When was Persepolis founded?
answers: 518 BC