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
Geography of Mesopotamia country, Nineveh, Kalaḫ and Arbela, were all on the east bank of the Tigris. The reason was its abundant supply of water, whereas the great plain on the western side had to depend on streams flowing into the Euphrates. Mesopotamia means "(Land) between two rivers" in ancient Greek. The oldest known occurrence of the name Mesopotamia dates to the 4th century BCE, when it was used to designate the land east of the Euphrates in north Syria. In modern times it has been more generally applied to all the lands between the Euphrates and the Tigris, thereby incorporating not only