The Sultan Ahmed Mosque, universally known as the Blue Mosque for the 20,000 Iznik tiles lining its interior, faces Hagia Sophia across the Hippodrome. Built between 1609 and 1616, it is one of the few mosques in the world with six minarets. Entry is free but the mosque closes to visitors during prayer times. Dress modestly and remove your shoes — headscarves are available at the entrance. The courtyard is as large as the mosque itself and worth lingering in.