The fortifications that surround the historic part of the city date back to the early 16th century. When Lucca grew bigger, instead of tearing down the walls as it's been done in so many cities, the fortifications were left entirely intact and the rest of the city simply grew outside the walls, which of course greatly contributes to the unique atmosphere you find in the historic part of Lucca. Now that the fortifications don't serve any kind of defense purposes anymore, a track has been paved on top of the 12-feet-high walls that makes for an awesome bike path or a really nice promenade, offering great views of both the city and the surrounding countryside.
I thought it was especially fun to be there on a Sunday afternoon, when both tourists and locals meet up for a lazy stroll around the city. Going all the way around the fortifications allows you to see the old city's six gates and 11 bastions, covering a total distance of about 4 km.