We stayed for a week in this lovely little apartment. It's super convenient to access central areas of Lisbon on foot or it's 10 minutes from the metro. (We always cut up by the Miradouro de Santa Catarina.)
Yes, there are 3 flights of stairs, but we are in our 70s and were fine.
The kitchen is well equipped but mind your head on the corner of the cupboard between hob and sink!
The pasteis de nata at the Manteigaria near the metro are superb!