Birtha (Mesopotamia) bastions are quite traceable. There are the ruins of a vaulted secret staircase, leading down from the heart of the citadel to the water's edge. Birtha (Mesopotamia) Birtha () was an ancient fortress on the river Tigris, which was said to have been built by Alexander the Great. It would seem, from the description of Ammianus, to have resembled a modern fortification, flanked by bastions, and with its approaches defended by out-works. Shapur II here closed his campaign in 360, and was compelled to retire with considerable loss. D'Anville and Edward Gibbon both identify Birtha as modern-day Tikrit, Iraq. The
History of Mesopotamia the Roman Empire. Christianity as well as Mandeism entered Mesopotamia from the 1st to 3rd centuries AD, and flourished, particularly in Assyria (Assuristan in Sassanid Persian), which became the center of the Assyrian Church of the East and a flourishing Syriac Christian tradition which remains to this day. A number of Neo-Assyrian kingdoms arose, in particular Adiabene. The Sassanid Empire and Byzantine Mesopotamia finally fell to the Rashidun army under Khalid ibn al-Walid in the 630s. After the Arab-Islamic conquest of the mid-7th century AD, Mesopotamia saw an influx of non native Arabs and later also Turkic peoples. The city