Next to the mosque (that we see in the distance) there is a playground. Under it are the remains of a "vast Roman rotunda built in the fifth century for some unknown purpose but never finished: it was evidently intended to have a dome, in which case it would have rivalled the Pantheon in Rome, for it was thirty meters in diameter. It remained unfinished for some five centuries and then in about 922 the Emperor Romanus I Lecapenus determined to use it as a foundation for a small palace. He had it vaulted over, supported the roof on 75 columns and built his palace on top. (The outline of the palace is marked in concrete on the terrace.)

Text from Strolling through Istanbul.