I'll start with the deal breakers of my stay then I'll list the good parts, so you can decide for yourself.
There is no heating in the apartment! it is not clearly stated in the description so you have to be extra careful and check for the amenities listed. I found that out after I entered the apartment which was really cold.
There are no windows in the house that opens to outside apart from the bathroom windows which opens into the semi-closed building stairs. So no fresh air, no sunlight inside the house since it was built. Not good for people who has allergic asthma (e.g. me). No outside view either, you need to guess the weather.
There were ants and small spiders in the bathroom and inside the bathtub. There are spider webs and some spiders in the bedroom, near the bed.
The shampoo in the unnamed bottle apparently not used for a very long time and mixed with sth, it went bad, so it was terribly stinky and impossible to use, so I had to use the shower gel to wash my hair. There are no hand towels either.
And here are the good parts:
The location is perfect!! It is right at the centre of Porto so you don't need any transportation other than from/to airport which is also super convenient to reach since there are regular bus and metro lines quite close to the house.
Carlos greets you in front of the building, and he is super communicative, he let me check-in in the morning as well. He provided some local tips and made sure that everything was fine.
He brought a heater into the room yet still, though the room became slightly warm afterwards, the rest of the common areas was still cold.
Overall, if you're travelling on a tight budget, I recommend his place since it's at the very centre. Yet if you can pay a couple of more euros, I would recommend you to stay somewhere else especially in winter.