The Hagia Sophia (also known as the Aya Sofya, which is taken from the Greek words for ‘Holy Wisdom’) is a unique Byzantine building and an icon of Istanbul. Originally the seat of the Patriarch of the Greek Orthodox Church, it was also used as a mosque by the Ottomans before being secularized and converted to a museum. The site is hugely popular and can be extremely busy, so book your tickets in advance to skip the lines!