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
Mesopotamia (theme) Mesopotamia (theme) Mesopotamia () was the name of a Byzantine theme (a military-civilian province) located in what is today eastern Turkey. It should not be confused with the region of Mesopotamia or with the older Roman and early Byzantine province of Mesopotamia. The Byzantine theme was located between the rivers Arsanias (modern Murat) and Çimisgezek. The theme was formed probably between 899 and 911, when Emperor Leo VI the Wise (r. 886–912) appointed the former "strategos" of the Charsianon, named Orestes, as its governor. Most of the province was formed out the Armenian principality of Takis, ruled by the chieftain