As others have said the location of the apartment is perfect to explore Porto. You can reach most sights by foot and if you need public transportation there are metro and bus stops nearby as well.
We found the flat to be very clean and felt very comfortable. For two people there‘s plenty of space. One thing to maybe consider is that it is an old building with wooden floors which means you can hear the neighbors above you walking around - although this did not bother us at all.
Communication with the hosts before and during our stay was good and the self check in was easy.