The cattedrale di S. Martino lies in a very nice square and shows an outstanding architecture. Do not miss the statues of S. Martino (both in the facade and in the interior).
Interior is very dark and disappointing in comparison to the exterior, but it is still a very fascinating church. When you visit Italian churches you may get used to such masterpieces but never forget they really are!!!!
I would advise photography attempts on the cathedrals and churches of Lucca, the architecture makes great contrasts with the sky, and quite simply you can take a little bit of these beautiful buildings home. Here is my attempt: