The Strolling through Istanbul guidebook pointed out that near the Anadolu Hisar there also is a namazgah, an open air mosque, with a stone mihrab and mimber within a walled plot. People there told me it was still in use. It seems to date from somewhere between 1660 and 1780.
I had never seen a namazgah before, curiously I saw another one only a few months later in Bodrum.
And half a year later it turned out there was one that I'd known for ages, but had never taken for more than a fountain. I put the pictures of that one in this gallery (maybe for the time being).