Sultanahmet is the old imperial core, and the good news for planning is that almost everything here is walkable. Hagia Sophia (Ayasofya), finished in 537 under Justinian, was the largest cathedral in the world for centuries before becoming a mosque in 1453. It is an active mosque again today: entry is free, you dress modestly, shoes come off on the carpet, and it closes to tourists during the five daily prayer times — so check the call-to-prayer schedule and aim for mid-morning.
The Blue Mosque (Sultanahmet Camii) faces it across the square, maybe a four-minute walk, recognisable by its six minarets and the İznik tilework that gives the interior its blue cast. Topkapı Palace, the Ottoman court for roughly 400 years, is a five to ten-minute walk north and needs a couple of hours on its own; the Harem is a separate ticket and worth it. The Basilica Cistern, a sixth-century underground reservoir with 336 columns, sits just across from Hagia Sophia and now charges a separate entry with a timed queue that moves fastest first thing.
If you only do one thing right in Sultanahmet, do it before 9am. By late morning the square fills, the Hagia Sophia line lengthens, and the heat in summer makes the open square unpleasant.




