A few steps from Sao Bento Station (which you need to visit for the wonderful Azulejos). You are more or less half an hour from the airport by metro and from the apartment you can walk and discover a lot of things around the city. Porto has been a great discover and such a fascinating city. Easy self check-in and check-out procedures, absolutely worth it!!
P.S. If you like vintage, you must save 2 hours of your travel to visit Armazém, an incredible vintage shop with a bar in it, I loved it very much!!