The one unmissable building in the first courtyard is the church of Hagia Eirene (Divine Peace) that was rebuilt by the Emperor Justinian in 537 after an older building was destroyed by the Nika Riot of 532. It was patched up again after an earthquake in 740, since when it has been little altered. After the Ottoman conquest it was enclosed inside the walls of Topkapi Palace and used as an arsenal by the Janissaries. Later it became a forerunner of the Military Museum in Harbiye (LINK).
Today it serves as an occasional concert hall but can’t be visited at other times, which is a shame as the sight of its great brick dome and its apse adorned with a simple mosaic cross is very dramatic. The mosaic itself is of uncertain date, some attributing it to the Justinian period, others to the tion in 740. The synthronon (bench seat ringing the apse) is the only such known in the city.
The first church on this site appears to have preceded the reign of Constantine the Great who probably had it rebuilt. Peaceful as the setting may seem now, it was here in 346 that more than 3,000 people died in a riot over one of those obscure theological squabbles for which early Christianity was renowned (in this case Aria-nism versus Orthodoxy).