Right across the plaza from the Blue Mosque, stands the Hagia Sofia (here the Ayasofya). The name means "Holy Wisdom" and the structure was the largest cathedral for nearly a millennium.
After the fall of Constantinople, the cathedral was converted into a mosque. You can see the evidence of the conversion by the large discs of displayed Arabic script and the adjustment of the nave to be tilted slightly offset so that the innermost portion of Aya Sofya points toward Mecca.
The cavernous interior is decorated with ornate painting and mosaics such as the one shown here.