Large attic apartment about 10 minutes walk from a metro stop which will take you right into the heart of Berlin (going through Checkpoint Charlie). The other reviews are correct when they tell you to be prepared for some stairs, but this wouldn't put me off going back there. We were a group of 4 and there was plenty of room. Lots of towels. Very clean.
Key collection and checking out was very easy. This is very important to me. We don't speak any German but this didn't matter at all.