I stayed at Katerina's a few weeks back and absolutely loved my time there. The location is quiet, but in a fantastic neighbourhood, not far from lively bars and yummy restaurants and a more than adequate supermarket. Syntagma (Constitution) Square is a twenty minute walk away through the Park, and the Acropolis is another 5 minutes on from there. I really recommend Katerina's apartment and Katerina herself is super friendly and always on hand with top tips, help and local advice.