Baghdad, of course remains the most famous memorial to the Abbasids,
although nothing remains of the fortified Round City built for the Caliph
Mansur (754 - 775). Its purpose was to serve as an administrative capital,
a Caliphal palace, and a place to settle thousands of Khurasani-Arab troops.
The location of Baghdad was also particularly good for communications.
Militarily it housed a large army at the center of the state; these forces
included regular troops, the Caliph's own guard, the city's shurta police
and haras internal security force, as well as Baghdad's own paramilitary
militia. The great majority of the population, which may have reached
as many as a half-million by the year 800, lived in sprawlingsuburbs
around the Round City, some of which had been built by Khurasani
military chiefs to house their own followers.
D. Niccole and Angus McBride(illus.), "Armies of the Muslim Conquest," Men-At-Arms 225, Osprey Pub., 1993, Pages 23-24.
The 'later tower' means that sometime since the fortification was
constructed, a tower was added to the curtain wall. It was
well known that as european forces came into contact with
muslim forces at a fortification, the Muslim engineers, borrowed
the technology that they saw in the improvements of their
opponent's fortifications .