Sancaktar Hayrettin Mosque, Byzantine monastery turned mosque in Kocamustafapaşa, Turkey
Sancaktar Hayrettin Mosque is a brick building with an irregular octagonal exterior and a cross-shaped interior layout. Its walls alternate between brick and cut stone courses, creating a distinctive appearance across its facade.
Originally built as a Byzantine monastery, the structure was converted into a mosque following the Ottoman conquest of Constantinople in 1453. This transformation reflects how religious buildings gained new purposes after the political change.
The building displays how Byzantine and Islamic architectural styles merged within a single structure, creating a visible blend of two religious traditions. Visitors can observe elements from both periods coexisting in the walls and overall design.
The mosque sits approximately 500 meters northeast of Kocamustafapaşa railway station near Teberdar Street. The surrounding neighborhood has numerous artisan workshops and small shops that give the area its everyday, lived-in feel.
The original Byzantine apse remains visible on the eastern side of the building today. This remnant from the original construction shows that the ancient structure was intentionally preserved when converted to new religious use.
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