Stay away, this is unsafe.
The apartment is very dark as it's practically underground. I found it a bit cold as well, and I can tell they tried to fix the humidity and mold issue reported in reviews by previous guests by installing dehumidifiers. Humidity was indeed not a problem and I didn't see any mold, but it was very noisy due to the machines running the whole time.
The fact that it's underground made it impossible to open a window and change the air, so it felt like it was just smelling bad at times. The only window there leads to some dodgy underground cellar that is dark, smells like damp and I was afraid might have rats inside.
The worst part is that while I was sleeping on my first night after a long trip, at about 5am someone unlocked the door of my apartment and came down the stairs. Fortunately it was just another guest that was looking for a different apartment, we both got startled by each other's presence, they apologised and left. But it's very worrying that they had the key and could just open and casually walk into my flat.
I immediately wrote to the host asking for an explanation and a quick fix (I expected they would install a lock inside the apartment or something equivalent). All they did was stall the communication with very slow replies, then they asked for a picture of the key, then for a second picture of teh key, blamed me for not locking my door (it's not true, it was locked and I woke up precisely by the noise of the lock opening), then generically blamed guests that don't follow their check-in instructions and just dropped the matter.
This is what I suspect: the key I was given opens both the front door of the building and the door of my apartment, numbered 1. This must be also true for apartment n. 2 that the other guest was looking for, ie they can open the front door and that to apt. 2 with their single key. Since both keys 1 and 2 can open the front door, then they must be the same key and so any key can open any flat's door. This sounds very dangerous to me as anyone could have broken into my flat and steal while I wasn't there. When I was there I made sure to put a chair behind the door to make it reasonably hard to open it from the outside but I still felt very unconfortable. Fortunately it was only for a few days, as I had nowhere else to go so I had no coice but stay.
On top of the very stupid idea of having the same lock for all doors, I find it very upsetting that the owner couldn't be bothered to give an apology or even a honest explanation or install an extra lock the next day, especially given the very high price I paid for my stay. I will make sure I'm not going to go there again. For that price I expected at least some basic safety.