The Hagia Sophia is the most famous building in all of Istanbul. It’s a marvel of 6th century architecture and it’s still a marvel today. Originally a Christian cathedral, it was converted to a mosque when the Ottomans conquered Istanbul. In the 20th century, the building was turned into a museum and in 2020 it was converted back to a mosque by Turkish president Recep Tayyip Erdogan. We visited during Eid when tourists and pilgrims lined up in a queue that looped around Sultanahmet Square.