First of all I had to give private information on a website I've never heard of before in order to arrange the check-in, then the keys had to be picked up at a tabacco store around 10min walking distance from the appartement. Due to a delay of our train the keys apparently were brought to that tabacco store from another location and the host tried to request 15€ because of that - after firm refusal the host suddenly stopped asking again, which to be honest seemed like a scam, since it's the responsibility of the host to hand over the keys and delays are very common during travel.
Right around the corner of our street there were some homeless people sleeping outside in the freezing cold, so it's not the best district, but it's still safe.
The apartment itself was fine - quite spacious and the heating worked very well.