The place is unique as you stay in a medieval town in an authentic, tiny (bear that in mind if you enjoy spacious accommodation) house.
Maria is a lovely host. Make sure you ask her for tips - she knows where to eat really good (although it seems that you can't go wrong on Sifnos).
Tip from a Cyclade-hopping tourist: rent a car, it makes Sifnos much easier to explore and Maria's place easily reachable at any time of the day or night.