sentences:  The Abbasids' seat of power for almost 500 years was Baghdad, a city considered to be the jewel of Islam and one of the largest and most powerful cities in the world.But under attack from the Mongols, the city fell on , a loss often considered in the Muslim world as the single most catastrophic event in the history of Islam, the end of the Islamic Golden Age.The Christian Georgians had been the first to breach the walls, and as described by historian Steven Runciman, "were particularly fierce in their destruction".