Founded by Alexander the Great himself, and later an important fortress on the front line between the Christian and Persian empires, Raqqa finally served in Islamic times as the summer capital of the Abbasid empire.  Raqqa’s circular plan mirrors that of Baghdad, the Abbasid capital farther south on the Euphrates, and the best-preserved remains of the original brick walls are appropriately found at Baghdad Gate.  Follow the walls north to the 9th century Qasr al-Banat, or Palace of the Maidens, for a rare example outside Iran of the elegant four-iwan architectural plan, and enter the city walls to visit the 8th century Great Mosque, reconstructed in 1165 by the Zengi leader Nur al-Din.