I can't recommend this place enough. The location is excellent, a few meters away away from the Clérigos church and tower and within walking distance from the Ribeira plaza by the Douro river, the Palacio da Bolsa, Bolhão and many other attractions.
The apartment itself is great. It has a large, charming backyard where you can cool off when you come back home on a warm day. It's spacious, quiet, and the mattress is excellent.
I totally recommend this place.