From My Travel in Tuscany:
I have to thanks a friend of mine that suggested me to get this unusual tour created by Turislucca, a group of experienced authorized tour guides with a great knowledge of the town, that told me tons of interesting facts about the history of their birthplace. If a “must to do” in town is a bike ride along the wide and shady path of the fortification that surrounds the historical center, the majority of tourists don’t know that the main bastions hide ancient underground to discover.
Stories of castles, fortress, and knights always fascinated me that the idea to explore the underground of the walls of Lucca made me very excited.
We booked few days in advance because there are only two or three walking tours of the underground every month and so far, they are having a huge success. Anyway, we saw people buying the ticket on site the day of the visit, so have a try even if you decide at the very last minute. Mostly of the partecipants were Italians but the tour is available even in english.