What's the must-see thing in Istanbul?

Walk into Hagia Sophia through the Imperial Door and the first thing that hits you is the scale — the central dome floats 56 metres overhead, and the gold mosaic tesserae catch whatever light filters through the forty windows at its base. The building smells like old stone and carpet and faint incense. It's cold inside even when Sultanahmet square is baking at 30°C in July. Since 2020 it functions as a mosque again, which means free entry (previously around 100 TRY as a museum), but it also means prayer-time closures five times daily and a requirement to remove shoes and cover shoulders. The trade-off is worth it. You're standing in a space that was the largest enclosed room on Earth for nearly a thousand years — completed in 537 AD, and the engineering still looks like it shouldn't work.

The thing most guides skip: look up at the deesis mosaic in the upper gallery. Christ's face is half-destroyed, the gold leaf is peeling, and somehow that damage makes it more powerful than any restored fresco in Italy. The Ottoman additions don't fight the Byzantine bones — the massive calligraphic medallions float against the gold like they've always belonged there. Mind you, the upper gallery has been closed to visitors intermittently since the mosque reconversion. Check the day of your visit. If it's open, take the worn stone ramp — not stairs, a ramp, because emperors rode horses up it — to the second floor. The marble thresholds are polished smooth by fifteen centuries of foot traffic.

Sequencing matters in Sultanahmet. Hagia Sophia opens after morning prayer — typically around 9am for tourists, though the building itself opens at dawn for worshippers. Get there by 8:45, join the non-prayer queue at the tourist entrance on the south side. You'll be inside within ten minutes. After thirty to forty-five minutes (that's enough — the space reveals itself quickly), cross the square to the Basilica Cistern, which opens at 9am. The Blue Mosque is directly opposite but requires the same shoe-removal ritual and tends to have longer queues for less visual payoff. Worth seeing, but not the must-see. Save Topkapı Palace for the following morning — it needs two hours minimum and a pre-booked ticket.

One honest caveat: the 2020 reconversion removed much of the museum infrastructure — interpretive panels, lighting rigs, the scaffolding that let you get close to mosaics. What you gain is the atmosphere of a living religious space. The call to prayer reverberating in that dome is something you feel in your chest, not just hear. What you lose is the curated museum experience. If scholarly detail matters to you, read Rowland Mainstone's structural analysis beforehand — the building rewards preparation. But even without it, standing under that dome and understanding what humans built in 537 AD lands without any explanation needed.