Turkey
This former church and mosque is unmissably beautiful and is, in my opinion, in many ways a more satisfying experience than Aya Sofya.
This ancient church is one of the few places where you can see original byzantine iconography. The Haghia Sofia notwithstanding.
It is small but perfect and well worth the treck out to see. May be best combined with seeing the Golden Gate in Yedikule Castle and then taxi up passing the Theodosian walls.
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