Agios Nikolaos, known as St Nicholas of the Roof because of its large, heavy-pitched top, was founded in the 11th century. Originally part of a monastery complex.
The dome and narthex (enclosed passage between entrance and nave) were added in the 15th century, together with the roof to protect against snow.
Still in use, as evidenced by the old priest who followed me around to ensure I didn't take any photos inside!