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.
Iraq
In the 6th century BC, Cyrus the Great of neighbouring Persia defeated the Neo-Babylonian Empire at the Battle of Opis and Iraq was subsumed into the Achaemenid Empire for nearly two centuries. The Achaemenids made Babylon their main capital. The Chaldeans and Chaldea disappeared at around this time, though both Assyria and Babylonia endured and thrived under Achaemenid rule (see Achaemenid Assyria). Little changed under the Persians, having spent three centuries under Assyrian rule, their kings saw themselves as successors to Ashurbanipal, and they retained Assyrian Imperial Aramaic as the language of empire, together with the Assyrian imperial infrastructure, and an Assyrian style of art and architecture.
In the late 4th century BC, Alexander the Great conquered the region, putting it under Hellenistic Seleucid rule for over two centuries. The Seleucids introduced the Indo-Anatolian and Greek term "Syria" to the region. This name had for many centuries been the Indo-European word for "Assyria" and specifically and only meant Assyria; however, the Seleucids also applied it to The Levant (Aramea, causing both the Assyria and the Assyrians of Iraq and the Arameans and The Levant to be called Syria and Syrians/Syriacs in the Greco-Roman world.
The Parthians (247 BC – 224 AD) from Persia conquered the region during the reign of Mithridates I of Parthia (r. 171–138 BC). From Syria, the Romans invaded western parts of the region several times, briefly founding "Assyria Provincia" in Assyria. Christianity began to take hold in Iraq (particularly in Assyria) between the 1st and 3rd centuries, and Assyria became a centre of Syriac Christianity, the Church of the East and Syriac literature. A number of independent states evolved in the north during the Parthian era, such as Adiabene, Assur, Osroene and Hatra.