Previous reviewers have already covered the main points of visiting Istanbul, so I'll stick to the specific angle of visiting the city on an intraday layover. In November of 2019, while flying to Israel from the United States, I used the connection in Istanbul to plug a gap on my travel résumé, if only a little.

I had under five hours to explore central Istanbul. That is about enough for a “scratching the surface” type of tour. I visited the Blue Mosque, Hagia Sophia, Basilica Cisterna, Nuruosmaniye Mosque (which has a separate tentative entry on the WH list), the Grand Bazaar, Suleymaniye Mosque, and the Galata Bridge and the surrounding waterfront area, interspersing that with stops at rooftop cafés for coffee, sweets, and views, as well as one döner stop. Topkapi Palace, unfortunately, could not fit into that timeframe, since I am told it requires at least a couple of hours by itself. I suppose overall that is sufficient to firmly consider the WHS site as "visited", but I certainly did not come anywhere near getting to know Istanbul on par with my familiarity with other great European capitals. I suspect that two or even three full days must be the absolute minimum to achieve that level of exposure to Istanbul.

The new Istanbul airport is located much further out of the city than the old Ataturk Airport was – the ride between the airport and the city takes no less than 40 minutes and can last over an hour, depending on traffic. The airport itself is humongous and may require a non-trivial amount of walking between its central entry/exit areas and the gates. Therefore, the earliest reasonable time you can get to the city center after landing is within about an hour and a half. And you need to consider starting your trip back from the city center no later than two and a half hours before your scheduled departure time. In other words, roughly four hours of any layover had to be given to the airport/transfer logistics.

If your time in the city is short and you want to visit paid attractions – such as Hagia Sophia or Basilica Cisterna – then having a licensed guide in your employ saves you the time of standing in lines (in addition to the obvious benefits of guidance and narration during your tour). The guide can also arrange for your airport transfer to and from the city, which helps with peace of mind.

[*updated*] In October of 2023, I finally managed to give Istanbul the attention it deserves and spent five full days in the city. I visited quite a few additional sites named in the WHS inscription, although somewhat ironically on balance that firmed up my initial feeling of my short first visit which included just a few highlights being probably good enough for the purposes of counting Istanbul as visited.

The mosques are all architectural wonders, but once you visit Hagia Sofia, the Blue Mosque, and the Suleymaniye Mosque, most others will feel a notch below (for instance, the New Mosque is significantly more awesome on the outside than inside); Zeyrek Mosque is the only other unmissable one, in my opinion, primarily because of its Byzantine lineage. To my chagrin, the Chora Church evaded my attentions, being closed for renovation during my visit.

Topkapi Palace offers plentiful insights into the workings of the Ottoman Empire, with a marginally overwhelming amount of information provided via audio-guides and stands. Once you go in, anything less than 3 hours means that you are skipping areas or rushing through some exhibitions. That being said, the huge complex lacks a visual wow factor. If it was not explicitly a major component of the WH inscription, I would probably rate it a grade below "must-see". Hagia Irene, accessed on a combo ticket with Topkapi, is barely worth a couple of minutes of exploration - an incredibly old church with nothing to see inside.

Topkapi, Basilica Cistern, Hagia Sofia, and the Blue Mosque all sport nearly intolerable lines from mid-morning to mid-afternoon. In the cases of the Cistern and the Blue Mosque, the lines thin out after 2 pm, but Topkapi and Hagia Sofia can seemingly be entered without too much wait only first thing in the morning. (For paid attractions such as Topkapi and the Cistern, you can pay extra for a "skip-the-line" access with a guide). With all other mosques, you just need to be cognizant of the intraday closures for services.

Overall, a little bit over two full days was enough for me to visit and re-visit all of the key sights in Sultanahmet, Zeyrek, and Suleymaniye quarters.