The apartment is located right at the heart of the city. Syntagma Square is 2' away, Plaka the same. Acropolis 10' walking and Monastiraki about the same.
Just have in mind, a hot spot means also people, bars etc., so expect some more sound activity than normal coming through the balcony during the night.
By the way, Maria is a great host, very polite and helpful. I recommend this apartment for short trips to Athens, either for vacation or work-related.