My mom (67) and I stayed at Chiho & Nuno's flat for 5 nights. We loved everything about it. The view was unbelieveable, the breeze through the open windows was lovely and the accessibility to Alfama and all of Lisbon was unbeatable. We certainly got our workout in each and every time we had to climb the three flights of 14 stairs up to the flat from the street level. By the end of our stay, we could power up those stairs no problem. You may want to pack light, though, as getting heavy suitcases up there was tiring!
We had an awesome time, thanks so great hosts and a great flat, and can't wait to visit again!