There are plenty! The Castro is centrally located in the geographic heart of the city- and public transit options are numerous: there are BikeShare stations and ZipRide (cars), the MUNI has many options. For $2/ride you can jump on the underground (it does connect with BART (the regional transit service)). On the street you have a choice of electric buses (24, 35, 37 to name three), take the historic F-line trolley that runs along Market Street downtown and up to Fisherman's Wharf. Where ever you want to go in the city, you're conveniently located to catch whatever transit will get you to your destination...
http://www.sfmta.com/getting-around/transit
