These are the remains of Saint Clemence, the Byzantine church of Ankara.
Saint Clemence was the patron Saint of the city. He was the metropolitan of Roman Ankyra during the reign of Emperor Diocletian.
The church was built somewhere between the 4th and the 9th century A.D.
During the Ottoman times, a mosque was built over the church. Both were burned in a fire in 1917.
Nowadays, only some walls survive, along with some of the marble revetment.
The urban sprawl of the Turkish capital has almost swallowed the remains. Let's hope something is done towards the preservation of this unique monument.