Karsten's apartment is quite huge, especially the bedroom. Plenty of space to put luggages. The whole place was very clean despite an old apartment. The bed was very comfortable, enabling us to rest well after walking whole day at Berlin. It was near the subway, which made traveling via public transport super easy. The only downside was that sometimes the internet would slow to a crawl but other than that, our stay there was great.
Karsten has been very kind towards us, allowing is to check in early and check out late which fantastic because we had early flight into Berlin and night flight out of Berlin. I would recommend others to stay here as well.