Persepolis
Darius I built the greatest palace at Persepolis on the western side of platform. This palace was called the "Apadana". 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 long with seventy-two columns, thirteen of which still stand on the enormous platform. Each column is 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 5 cm, which was used for bonding, and then covered with the greenish stucco which is found throughout the palaces.
Achaemenid architecture
Persepolis was by no means the only large scale Achaemenid project, as Susa also hosted a similar structure initiated by Darius for similar ceremonial purposes. However, that history is able to enjoy the remains of Persepolis as opposed to meager remnants of Susa, owes partly to selection of stone in construction of Persepolis over mud-brick in Susa, and the fact that it had been relatively uninhabited, protecting it from wear and tear of inhabitants. Politically, Persepolis also was a significant find because nearby discovery of Naqsh-e Rustam, the Persian necropolis home to Darius the Great shed light on the significance it has had as one of the major capitals of the empire. Naqsh-e Rustam would not only house Darius the Great, but also his son Xerxes the Great, Artaxerxes I, and Darius II as well. The necropolis complex was looted following invasion of Alexander, and possibly in the Sassanid period and during the Arab invasion.