We really enjoyed Berlin! Staying at Katja’s was a great experience, considering public transportation in Berlin is really efficient, its location is really good and the neighborhood felt really safe (even when we walked back around 5 am one day).
The building has an elevator and a the apartment has little terrace that’s amazing for a coffee. Bathroom is great and the room is perfect for a couple. I suppose it can be a little chill during the winter but it has its heating system.
I’d really recommend you stayed at Katja’s if you want to get to know the city with your couple and feel really safe but near to public transportation (tram and metro).