Many companions of Muhammad lived in Raqqa. In 656, during the First Fitna, the Battle of Siffin, the decisive clash between Ali and the Umayyad Mu'awiya took place about west of Raqqa. The tombs of several of Ali's followers (such as Ammar ibn Yasir and Uwais al-Qarani) are in Raqqa and have become sites of pilgrimage. The city also contained a column with Ali's autograph, but it was removed in the 12th century and taken to Aleppo's Ghawth Mosque. The Islamic conquest of the region did not disrupt the existing trade routes too much, and new Byzantine coins continued to make their way into Raqqa until about 655-8. The Byzantine government may have seen the area as just temporarily in rebellion. Byzantine coinage probably continued to circulate until at least the 690s, if not even longer. Raqqa appears to have remained an important regional center under Umayyad rule. The Umayyads invested in agriculture in the region, expanding the amount of irrigated farmland and setting the stage for an "economic blossoming" during and after their rule. The strategic importance of Raqqa grew during the wars at the end of the Umayyad Caliphate and the beginning of the Abbasid Caliphate. Raqqa lay on the crossroads between Syria and Iraq and the road between Damascus, Palmyra and the temporary seat of the caliphate Resafa, al-Ruha'. Abbasid period In 770-1 (155 AH), the Abbasid caliph al-Mansur made the decision to build a new garrison city, called al-Rāfiqah ("the companion"), about west of Raqqa as part of a general investment in strengthening the empire's fortifications. The most critical part of this project was to secure the northwestern frontier with the Byzantine Empire, and al-Rafiqah was its largest and most important construction. It also happens to be the only one that survives to the present day. Although most of the interior layout of al-Rafiqah has since been built over, and much of its fortifications have also been demolished, about of its massive city walls are still standing, as well as its congregational mosque – the first in the world to be built from scratch on "a coherent, integrated plan" and a major influence on later mosque architecture. Although al-Mansur had conceived the vision for al-Rafiqah in 770-1, it wasn't until the next year that construction actually started. The caliph sent his son and eventual successor al-Mahdi to personally supervise the construction of the new city that