This superhip (but not studiously so) little watering hole is set in a colonial arcade that's said to have a hundred doors (hence the name) and is sometimes closed off for live bands. Bring a Sharpie (or ask them to lend you one!) and scribble a note.
The bar and the alley are both called Las Cien Puertas. Little bars spring up in this area all the time – it's one of the few places in Zona 1 you can really go bar-hopping, with dozens to choose from in this alley alone.