Ancient Şengül Bath (Ancient Roman Cistern)

Tucked inside a nondescript car park in Istanbul's Cağaloğlu district, this so-called 'layered building' compresses four distinct historical eras into a single structure: a Roman cistern at its foundation, Byzantine stonework above it, an Ottoman bathhouse known as Şengül Hamamı in the middle layers, and a modern Turkish Republic facade encasing the whole thing. It is one of the few places in Istanbul where you can read the city's entire architectural biography from bottom to top in one glance. Entry is free and unguarded — walk in through the south gate of the parking lot and the structure appears on your right. A handful of visitor reviews and the resident cats are your only company.