Ann B.
Yelp
We decided to visit the "Little Hagia Sophia" (so called because it was also built by Justinian, along similar lines to the Hagia Sophia) before visiting some of the larger and more famous buildings. It is certainly less spectacular than the Hagia Sophia, but it has something to offer for people who are interested in older buildings. Do not expect original Byzantine architecture, however, as it has been heavily modified over the centuries, most recently to brace it against the vibrations from the railroad which runs right next to the building.
For one thing, it is far less crowded, and you can take your time examining the paintings which decorate the upper galleries without feeling pushed or rushed. Although we did not have a language in common, the caretaker ensured that we saw all public parts of the building, by inviting us to climb to the gallery when we otherwise wouldn't have done so.
As with all active mosques, you need to take off your shoes before entering, and women are asked to wear a headscarf. There were headscarfs you could borrow just inside the door. There is no charge to visit.