Hidden byzantine gems to visit in Istanbul

Hello dear Community,

I will visit Istanbul for a few days and I would like to visit some byzantine sites in the city. Apart the broadly known touristic things (i.e. theodosian wall, hagia sophia, serpent collum) I would like to ask you: Are there any so called hidden gems? I am in particular interested in the siege of 1453, the byzantine architecture and warfare.

Thank you very much!