The Sultan Ahmed Mosque is located in Istanbul, the largest city in Turkey and the capital of the Ottoman Empire from 1453 to 1923. It is regarded as one of the greatest masterpieces of Islamic architecture. The Blue Mosque is one of the most beautiful mosques in the world and one of very few to have six minarets. It was built near the Hagia Sophia, over the site of the ancient hippodrome and Byzantine Imperial Palace (whose mosaics can be seen in the nearby Mosaic Museum).The other striking feature of the exterior is the beautifully-arranged cascade of domes that seem to spill down from the great central dome. The arcades running beneath each dome add further visual rhythm. None of the exterior is blue - the name "Blue Mosque" comes from the blue tiles inside. Inside, the high ceiling is lined with 20,000 blue tiles that give the mosque its popular name. Fine examples of 16th-century Iznik design, the oldest tiles feature flowers, trees and abstract patterns. The Blue Mosque was commissioned by Sultan Ahmet I when he was only 19 years old.