We had a great time at Sérgios flat. We arrived really late, but Sergio waited up and picked us up from the metro station to bring us to the apartment (without charging a late checkin fee).
We had everything we needed and right away turned on the little heaters he had for us. We've been to Lisbon in feb. and it can get pretty cold in Lisbon during that time.
Nice double bed, small bathroom, well equipped kitchen (also some cookies, water, tea, salt, sugar and olive oil and even a small gift for us). :)
The apartment is very central and close to the castle and it is very quiet! Barrio alto is reachable by 15-20min walking. And don't forget that the apartment is uphill! Late check out wasn't a problem!
Have fun at Sérgios place, it really is quite nice for couples! I'm sure its very nice in spring and summertime!