Castelli Toscani is a website about castles in Tuscany (http://www.castellitoscani.com). Somehow I keep coming back to this site as I search the web for things to do and new places to visit in Tuscany. It is very well done and has a ton of interesting information and original photographs. If you are planning on visiting the countryside around Florence and are interested in castles and fortresses, visit this site beforehand. They also have a page about the walls and gates of Florence.