Istanbul is the only city built on two continents, and the split is not a technicality — the ferry between the European and Asian sides is a twenty-minute commute that thousands of people make twice a day, and the two halves feel genuinely different.
It has been the capital of the Roman, Byzantine and Ottoman empires in turn, under three names, and all three are still standing on top of each other. You can be inside a sixth-century church that became a mosque, then a museum, then a mosque again, and be out on a street of phone shops ninety seconds later.
Who it suits
People who like cities that are hard work. Istanbul is sixteen million people, hilly, loud, and laid out by nobody. It rewards walking and punishes schedules. If your best travel memories are of getting lost somewhere dense, it will be one of your favourite cities on earth.
People who eat. The food is the reason a lot of people go back — and the interesting eating is breakfast, street food and the meyhane tradition of small plates and rakı over several hours, not restaurants with views.
People on a budget. It is substantially cheaper than any comparable European city, and the gap has widened.
Who it does not
People who want a compact, tickable centre. The famous sights cluster in Sultanahmet, but treating that as the city is like treating Times Square as New York. The trip is worse if you stay in the historic quarter and never cross the water.
People who dislike being approached. Around the main sights you will be spoken to constantly by people with something to sell. It is not aggressive and it is relentless.
People who need a short trip to feel complete. Three days covers the monuments. It does not cover the city.
When
April to June and September to November. Summer is hot and full; winter is grey, wet and cold off the Bosphorus, and it is also when the city is at its most itself and its cheapest.
The practical shape of it
Fly to Istanbul Airport on the European side or Sabiha Gökçen on the Asian one — check which before booking, because they are two hours apart. The metro and tram network is good and the ferries are the best transport in the city and cost almost nothing.
Four days is the honest minimum. A week is better, and most people who give it a week end up planning a return.