I'll start with the summary: it's an amazing flat, very well equipped and located perfectly - right in the middle of the center.
We were met by Vitor, who thoroughly showed us the flat and was very hospitable. Though famous tram 28 is going just near the house, it's quite at night and during the day. A flat is rather spacious, I think it'll easily have 4 guests.
The flat is located on the first floor (which is the second from the ground floor) and you can either use stairs or an elevator. The flat is located about 5-7 minutes by feet from Arco da Rua Agusta and about 10-15 minutes from first stop of tram 28.
An entrance greets you with beautiful azulejo tiles. The location is in flat Baixa district so it was very comforting to return there after walking all day on steep hills of Alfama, Mouraria or Bairro Alto which are just nearby.
In Lisbon you can walk on flat surface and just around the corner there'll be a couple of steep long stairs.
When I return to Lisbon again, I would like to stay in this flat again, as it was so amazing and had a major influence on overall impression of Lisbon.