I stayed here with 3 other friends. Marilena's home in Kastro is really nice, clean, beautifully located and spacious for all 4 of us. Just a few steps outside, there's a beautiful view from the top of Kastro overlooking the water, churches, and homes along the edge of Kastro.
The home is located as advertised at the top of the hilly Kastro so be prepared to walk up a lot of stairs. The homes are lovely and there are plenty of cute homes, walkways, and cats to distract from the walk. There are 2 great restaurants near the house, one a minutes walk away and a fisherman's restaurant down by the water.
We rented a car to get around and see as much of Sifnos. Highly recommend this over relying on taxis n the bus.
Would highly recommend a stay with Marilena. She was always available and helpful with check in since our group arrived at different times.