We found this cistern, thanks to the Rils. It's good that you can watch it for free and understand what kind of structures they are. The tank itself has models and signs with information about the nearby racetrack. The entrance is through a carpet store - we were lucky and no one tried to sell us anything at all. There is an inscription on the door that says that the entrance to the cistern is by appointment and only for the day - we passed quietly and no one told us anything either. There were a couple of people in the tank at that moment.
It's not a big tank. It's interesting inside! Added various exhibits inside! A cistern is a place where water supplies for the city were stored in ancient times! There are several such places in Istanbul, there are paid ones, there are free ones! This church was visited because of the proximity of the accommodation, there are free larger-scale ones. There are even more large-scale paid ones!
A wonderful, small cistern, without water, but renovated. Enough to understand how water was collected in Constantinople. The carpet shop upstairs is also worth your attention, a lot of interesting things, a very atmospheric place.