The apartment is nice and fits easily up to 4 people. It's central and you can easily walk to all main things in Lisbon, in particular the castle and the first metro stop (Martim Moniz) are less than 10 mins away. The area is multi-ethnic and with a lot of small shops. By night it might get a bit dodgy, depending on the day of the week. There is a very nice small terrace with a beautiful view of Lisbon, and the view from the two rooms facing the street is also nice. Consider that one room (the one with the single bed) has no windows on the outside, only a small window close to the ceiling that opens on the internal corridor.
Hugo is really nice and will help you during your stay. We only had a minor hiccup during the check-in when he was late but we thought it was an exception. A couple of downsides of the apartment have to be considered: the kitchen is old, with old appliances, so it's hard to do anything anything more than preparing a coffee or tea. However, there is a table for 4 people and enough room to sit together. Finally, in the morning and evening there was a smell coming from the bathroom and kitchen, probably due to old sink pipes not properly blocking the odour. A clean-air spray is provided, but opening the windows is needed sometimes anyway. Consider this if coming during winter, as there is no heating system. However, we stayed for one week during November and this was never a big issue, the climate is warmer than rest of Europe, the house stays warm and we slept really well. All in all we liked our stay.