According to this – http://en.wikipedia.org/wiki/Topkapi_Palace – Topkapi Palace in Istanbul, for five centuries the seat of the Ottoman sultans, was begun in 1459 on order of Mehmed II. But why did he have a new palace built? Why not use the old palace of the Byzantine emperors? And – what happened to that old imperial palace complex? Was it demolished?
The old Byzantine palace complex was at Blachernae. I’d assume Mehmed didn’t stay there because it wasn’t militarily defensible. It had fallen to the Crusaders during the Fourth Crusade, and then again when the Turks took the city. Topkaki was much more defensible.
Given that the Byzantines were Christian, maybe the old palace didn’t have proper accomodations for multiple wives and a harem. 
Because Istanbul was Byzantium
But now it’s Istanbul, not Byzantium
If you want the Imperial in Byzantium
See Topkapi in Instanbul…
The Blachernae Palace had been next to the Kerkoporta, the point where the Turks first breached the city walls, and it had been very badly damaged in the fighting. To have used it would have required a complete rebuilding. Building a new palace from scratch on a new site was therefore not that big a deal.
The Istanbul was originally Byzantium :dubious:
http://topkapi-palace.org/
It’s good shit.