Response from Ju:
Happy you enjoyed your stay in Paris!
You chose to take the eastern metro station (chateau rouge), not the nearest, to move around. It's (email hidden)ly place known as the African area of Paris. Most of my guests liked this place. Try the typical market on every saturday!
Metro 12, Lamarck caulaincourt is the nearest metro station and is in the west of the apartment (towards the Sacré Coeur) wich you said "is really nice".