You're literally right next to the Porto Cathedral (Se do Porto). So, you can check one must-see location off your list right away! Host was very responsive. I got to Porto early (7am) and asked them if they could let me check in early if possible. I didn't have much hope since check out is at 11am, and they still have to clean. But the hosts were available early in the morning, and they got back to me in an hour to tell me they'll make a special exception since there's a room available. (I spent the time eating a pastel de nata a few streets away)
Also, when they realized they didn't put the remote control in the room (I didn't even realize), they texted me to try to drop it off. Very attentive.
Place is walking distance to everywhere, very centrally located. Note that if you access the apartment from Sao Bento station, there's quite a bit of up/down. There's a heater for those coming in during the winter.