Hagia Sophia was completed in 537 and remained the largest enclosed space in the world for almost a thousand years. It has served as a cathedral, a mosque, a museum, and a mosque again, and each of those lives left something behind that was never removed.
That is what makes the interior unusual. Byzantine mosaics survive above the gallery level; below them hang the great calligraphic medallions added under the Ottomans. Both sit under the same dome, which appears to rest on nothing because of the ring of windows at its base.
Entry is free and modest dress is required, with heads covered for women. Visits stop during prayer. From Hotel Sultania the walk takes about ten minutes, and going straight after opening is the difference between a crowded visit and a calm one.