At one time, there were many towers inside the walls of Lucca. They were mainly the property of the rich and they served several purposes. First, one could see the enemy coming from high up in the tower. Then once the enemy was in the city, one could climb to the upper floors of the tower for protection. There were no stairs to the upper floors, just ropes. I guess they were only used in extreme circumstances. You wouldn't want to be on the 3rd floor and remember you had to go out for a loaf of bread...
Most of the towers are gone now but their bases were used to construct new buildings. One can still see the outline of filled in gates and windows. That gives an idea of how old the buildings are.
The wall around the city underwent changes over time mostly due to the changes in weaponry. Higher walls to protect from archers, then lower, thicker walls to protect from cannons. When changing the walls, the Luccans re-cycled the bricks and stones from the old walls so one can find remnants of Roman walls in the walls from the middle ages.
There are passageways under the walls leading to areas where soldiers would wait for attacks or ammunition was stored. Now these areas are being re-claimed to make a walkway for pedestrians to enter the city from the parking lots outside the wall (cars are very limited inside the wall).
There are 140+ churches in the small town. The main reason is that the nobility would leave their land, etc to the oldest son. The other children would have to sort of fend for themselves. So, quite often, the nobles would establish a church and a convent so that their un-wed daughters could have a place to live and spend the rest of their lives.
Speaking of churches, while excavating under a church, they came across a Roman house. There are probably many sites like this under churches and buildings all over town. We visited a Domus Romanus (Roman House) under a private home where we ate a lunch of typical Roman food of the era.
The Amfitheatre area can be a hot spot in the tourists season. It is built on the site of the original Roman Ampitheater. It is mostly shops and restaurants now.
Lucca is the birthplace of Puccini. To honor their most famous citizen, there is an opera recital every night of the year in one of the churches. During the low season it is in a small church but moves to St Martin church in the summer. For someone like myself, this recital was perfect... a few things I recognized and it only lasted an hour. It was one of the highlights of the tour. Unfortunately my camera ran out of juice before the recital started.