Nickolas' place is really nice, located in Kastro old town it is close to restaurants and a couple of mini markets.
The host was very kind and very quick in providing information of what to do and where to go in Sifnos, he even gifted us with a book about the island.
If you are staying in Kastro I highly recommend to hire a car because the bus to Apollonia leaves every 2 hours and the main beaches can only be reached by Apollonia or Kamares.