Istanbul: Sultanahmet - The Blue Mosque

The mosque (built 1603-17) is the masterwork of Ottoman architect Sedefkâr Mehmet Aga.
It's built on the site of the Great Palace of Byzantium, on the south-eastern side of the Hippodrome.
With its six minarets and a great cascade of domes, the mosque is a worthy sibling to Hagia Sophia