The Round City of Baghdad was centrally planned as a residence of Abbasid Caliphs and its core was built in 762-766, in just 4 years. Pre-planned structures, both grand palaces and mosques as well as various commercial and residential buildings were designed by the best architects and craftsmen of Islamic world at a time.
It took a shape of a circle with a radius of about 1 km. Great Palace of the Golden Gate as well as Grand Mosque of al-Mansur stood in its center, surrounded by the massive park and some selected institutional buildings. Two rings of the inner walls delineated various dense commercial-residential quarters.
The city was also a hub for knowledge, and had a large number of schools (madrasas) and libraries, including the great House of Wisdom, probably the largest Medieval library in Islamic World.
The birth of the city coincided with Islamic Golden Age, and destruction of Baghdad by the Mongols under Hulagu Khan in 1258 marked the dusk and decline.
It is said that when Mongols were sacking the city, river of Tigris ran black from the ink from the countless manuscripts thrown into the water, many of which were the rarest and maybe the only surviving copies of some translated Greek and Roman texts.