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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Go to the bus terminus at Eminonu on the waterfront side of the street.
Find the bus stop for number 90.
It should say Draman on front.
Ride the bus all the way to the terminus. It goes through a fascinating neighbourhood. Then continue to walk straight on up the same street up the hill.
At a T junction go left and immediately right up Nester St.
The church, a brick building which looks like a mosque is over on a side street to the right.
Don't go on Wednesday - it's shut.