Close to Republique, which puts you pretty much everywhere in Paris, by metro; and walking distance from the Centre Pompidou with, I would say, the most amazing view over the city.
Caroline's place is the right spot to spend some days, and if you're lucky enough, to be welcomed every day by a glorious sunshine that only that 6th floor can give you. Hard to believe that you get all that quietness right in the city centre. : )