The apartment is just wonderful! It is perfectly situated in Bairro Alto, so one can explore the city all by foot and though it is in a very lively area, it is very quiet to sleep at night. The apartment itself is very nice - I would immediately move in if I could ;-) - it offers everything you need for a comfortable stay and is very clean and very well maintained.
Duarte was a very nice host. He gave us useful tips for our stay in Lisbon and he really wanted us to have a nice stay. He was available for questions and answered right away.
Thank you very much! I will recommend Apartamento Sta Catarina to my friends and relatives :-)