The Sultan Ahmed Mosque, also known as the Blue Mosque, is one of the most impressive monuments in Istanbul, Turkey. Sultan Ahmed I commissioned the royal architect Mehmet Aga to build the mosque after wars depleted the treasury. Construction began in 1609 on the site of the Byzantine emperors' palace and Hagia Sophia, using some of their foundations. The interior features over 20,000 handmade tiles, more than 200 stained glass windows, and the dome and half-domes are decorated blue. It remains one of Istanbul's most significant historic and artistic sites.