Most people give Kaleiçi about ninety minutes — enough to photograph Hadrian's Gate, walk down to the harbour, and leave. It's not wrong, exactly, but it misses most of what makes the old town worth visiting in the first place.
Start at the top, not the gate
Instead of beelining for Hadrian's Gate, start near the Yivli Minare (Fluted Minaret) and let the lanes pull you downhill toward the harbour. This way you're walking with the old town's natural slope rather than climbing back up it at the end of a hot day, and you'll pass far more of the restored Ottoman houses that don't make it onto the standard photo route.
Budget time for getting lost
Kaleiçi is small enough that getting lost costs you minutes, not hours, and the best courtyard cafes and quiet corners tend to be the ones you find by accident rather than the ones with a queue outside. Build in at least half an hour of no fixed plan.
Go back in the evening
If your schedule allows it, see Kaleiçi twice: once during the day for the views and the museum, and once after the day-tour groups have left, when the lanes empty out and the harbour restaurants light up. It's a genuinely different old town in the evening.
For the full breakdown of what to see, read our Kaleiçi old town guide.