The Hagia Sophia was once a Christian church, but converted to a Mosque when Constantinople fell. During much later enovations incredible Christian mosaics were found and it was converted into a museum.
The Hippodrome was the Ancient Roman chariot racing circuit. Only this part remains the seating has long gone. The 2 pillars you see are original.
Istanbul is a lively city and if it’s your first visit I can recommend staying close to Sultanahmet Square as it is very close to the top attractions of Istanbul. Within a few minutes walk there are The Blue Mosque, The Hagia Sophia, the Hippodrome, The Basilica and within 10/15 minutes the Iron Rings Monument, the Grand Bazaar, Spice Bazaar and the riverBosphorus. After that, you’re on your own!
The Markets of Istanbul
As well as the well know grand Bazaar and the Spice Bazaar, the streets around these teem with shops and restaurants and are well worth the walk just wandering from street to street, or more exactly, alley to alley!