The Sultan Ahmed I built the Blue Mosque of Istanbul between 1609 and 1616. The sultan was only nineteen and wanted to build a mosque to compete with Hagia Sophia, which stands right across it. While the mosque's exterior is breathtaking with its massive dome, eight secondary ones and its six minarets, there is not a hint of blue in its exterior color. However, the interior is made of 20,000 blue tiles lining up to the ceilings and 260 windows. The Blue Mosque is a true melting pot of Islamic Ottoman architecture and Byzantine style of design.