Istanbul spans the Bosphorus. The historic peninsula is on the European shore, and Kadıköy is across the water. Base yourself by the sights and crossings you’ll use most: Sultanahmet for the monuments, Galata for the Golden Horn, Kadıköy for the Asian side.
In Sultanahmet, Hagia Sophia, the Blue Mosque and Topkapı Palace are a short walk apart. The T1 tram runs from here to Sirkeci, Eminönü and Karaköy. The streets around the big sights are crowded by day.
Sirkeci and Eminönü are where you catch ferries and the Marmaray, with the Spice Bazaar there too. North of the Golden Horn, the streets of Karaköy and Galata climb steeply to Galata Tower. Beyoğlu carries on up İstiklal Avenue to Taksim and stays up later than the old city.
Kadıköy has markets, restaurants and ferries back to Eminönü and Karaköy. Stay there if you’ll spend time on the Asian side or fly from Sabiha Gökçen. Any day in Sultanahmet then starts with a crossing.
Along the Bosphorus at Beşiktaş and Ortaköy, you’re close to the palaces and ferry piers but farther from the historic peninsula. Come in spring or early autumn for long walks and ferry rides. Winter is wetter and cloudier, and midsummer can feel humid.