The first and not only feature of Lucca are the intact Renaissance city walls that enclose the historic center (about 2.7 ml), the only example in the world of such dimension.
The walls have never been used for a defensive war but they defended the town center keeping it quite intact in its original aspect.
But not only the walls make Lucca unique, there are also its churches (about 100 consecrated and deconsecrated), towers and steeples and Renaissance palaces. There are also suggestive places as the famous amphitheater, born on the ancient Roman one, the squares and Fillungo street that by itself is worth a visit.
We can’t say no more, because there are countless publications about its beauties, both books and web (see for example wikipedia/Lucca).

Only another note:
just for its huge artistic treasures the town center of Lucca has been proposed to be included in the UNESCO World Heritage list.

A curiosity:
Up to 1860 or so, Lucca had a river port leading to the Tirreno sea.

Do not forget the gastronomic products:

• focaccia with Lucca oil
• buccellato, sweet aniseed bread with raisins
• spelt soup
• tortelli stuffed with meat
• biroldo and mallegato, made with pork blood and plucks
• castagnaccio cake made with chestnut meal
• pasimata, Easter cake
• torta co' becchi, short pastry (generally with chocolate, or cream or herbs) with a short pastry ring shaped as “horns”
• biadina, herbs strong drink
• china Massagli, herbs strong drink
• Colline lucchesi, DOC wine
• Montecarlo, DOC wine
  
  And the dainty mushrooms of the neighboring Garfagnana

Thanks to Silvia Frediani, expert on tropical freshwater fishes, for the beautiful pictures