Bulgarian St. Stephen Church stands out along the banks of the Golden Horn. Locals often call it Demir Kilise, or the "Iron Church," because it is built almost entirely from cast iron. This remarkable structure is one of the few surviving examples of a cast-iron church in the world.
In the late 1800s, the Bulgarian community in Istanbul wanted a place of worship separate from the Greek Orthodox Church. After splitting from the Greeks, they first built a wooden church in 1870. Unfortunately, that original church was destroyed by fire. When it came time to build again, there was a challenge: the ground near the Golden Horn was too soft for heavy stone or concrete buildings.
Instead of stone, the Bulgarians chose cast iron. Prefabricated iron sections were produced in Vienna, then shipped down the Danube and across the Black Sea to Istanbul. These pieces were assembled on-site like a giant puzzle, and by 1898, the new iron church was complete and open for worshippers.
After more than a century, the Bulgarian St. Stephen Church underwent a careful renovation, reopening in 2018 with its shining ironwork restored. Just across the street, however, the old archpatriarchate building tells a different story, its empty shell a reminder of the area’s layered history.
Source - https://en.wikipedia.org/wiki/Bulgarian_St._Stephen_Church