We had an excellent stay at Carsten's place. Everything was as good as advertised. Carsten communicated quickly with super helpful information. He made it easy to reach the place from airport via public transit and answered all questions we had.
We had plenty of room for two people and although there is room to sleep four people, that would have been tight. Something to consider for groups of 3-4.
As many people have mentioned, there is a significant climb of steps to the top. I mention this because I was in town for the Berlin Marathon. I would definitely stay here again for a regular visit to Berlin but I do not know if I would stay here while running the marathon. Thankfully we stayed elsewhere after the race but the stairs would be a brutal climb to do on tired legs.
Lastly, bring your own laundry detergent/pods if you plan to do laundry. There are dishwasher pods but we did not find any laundry detergent. Maybe this was a miss on our part but we looked everywhere.