We stayed in Sebastian's apartment for 3 nights. We are 2 adults & 2 teenage kids. The apartment itself is great. 2 double beds (with 2 single duvets on each) plus a double sofa bed. Very nicely decorated (all ikea). 4 flights of stairs might be an issue for some guests but wasn't a problem for us. Not in the most salubrious area but literally 50 metres from wedding ubahn station & from there 10 mins to central Berlin. We would stay there again.