Fun place - if you're looking for a no frills apartment that's close to all the tourist attractions in Lisbon this is a great spot. Although it's located in Bairro Alto which has a very active nightlife, the street this apartment is located on only has restaurants so it's a bit quieter than the other streets. The windows are well insulated so noise that reaches into the apartment is minimal if you keep the windows closed.
The neighborhood of Bairro Alto itself is on a hill so you'll end up having to climb up to this apartment at the end of the day, no different than any other apartment in Bairro Alto though.
As others have noted, you can hear other guests coming into the building and into their apartments, however I did not hear other guests once they were out of the stairwell.
The pictures in the listing are not of the exact place you'll get but I didn't find the differences to be significant.