Fritz's apartment is great, nice and open and light.
Very minimally fitted out, but still felt cosy.
It's about a one minute walk from the Sonnenallee stop, and just around the corner from lots of food options and supermarkets.
Like other reviews have mentioned, the stairs are a bit tough, they really get your heart going by the time you get to the top, but the sunrises and sunset views are well worth it.
We didn't meet Fritz in person but he had an English speaking friend meet us who was lovely and very helpful.
We'd definitely recommend this apartment.