Istanbul has 39 districts. You need six. Where to sleep and how to split the days comes down to three bits of water.
The Golden Horn cuts the old peninsula off from Beyoğlu. The Bosphorus cuts Europe off from Asia. The Marmara holds the islands.
The historic peninsulaSultanahmet, Eminönü, Fatih · the open-air museum, walled since the 400s, UNESCO-listed since 1985.
BeyoğluGalata, Karaköy, Pera, Taksim · "Pera" means "across" in Greek.
The Bosphorus villagesOrtaköy, Arnavutköy, Bebek · some 620 mansions on the water, no metro on the shore.
Kadıköy and ModaAsian side · the everyday city, and the neighbourhood travellers ask about most.
Üsküdar and KuzguncukAsian side · the old road to Asia, now a ferry and Marmaray stop.
Balat and FenerGolden Horn's south shore · home to the Ecumenical Patriarchate since about 1600.
The Princes' Islandsnine, four inhabited, no private cars, about 90 minutes by ferry.The "Asian side" sounds new. It's the old bit: Kadıköy began as Chalcedon around 685 BC, long before the monuments across the water.
So where do you sleep? Travellers keep landing on three bases, almost tied: Sultanahmet, Galata, Kadıköy. Two or three nights? Peninsula, and walk. Longer? Split it: Europe first, then Kadıköy. One traveller with six nights asked exactly that; the thread ran to 46 replies.
Split the days by water too: peninsula, then across the Golden Horn, then across the Bosphorus.
Cross the Bosphorus by ferry, on purpose. The T1 tram drops you at Eminönü pier; the crossing to Kadıköy takes 20 to 35 minutes, depending on the boat. Go midday, sit outside, watch the zones line up behind you. Times change by season; check on the day.