The Fortress of Saint Paolino (best known as the "Fortress of Ripafratta") is part of the historic-architectural heritage of the municipality of San Giuliano Terme (province of Pisa). Built during the Early Middle Ages, it is located in the small community of Ripafratta. It has for many years now been left abandoned and decaying, but its charm remains intact.

It is undoubtedly the symbol of the city, famous all over the world for its slope. The tower, which is the bell tower of the cathedral, has a cylindrical shape with blind arches on columns. Its construction began in 1173 and, after suffering many interruptions, was completed around 1370. Even before the third floor was completed, the monument began to lean to one side due to the sandy soil on which the foundations rest, deep just three meters. The slope reached five meters from the vertical at the highest point, but, following the long restoration works which began in 1990 and ended in 2001, during which impressive systems of lead counterweights (up to 900 tons) were used, and consolidated the base, it was possible to decrease the slope up to 3.99 meters; value which, in the opinion of experts, should remain unchanged for at least three hundred years. The 294 steps of the spiral staircase lead to the top of the tower through seven floors, six of which are surrounded by a small loggia with round arches that recall the motif of the facade of the cathedral. From the top you can enjoy a splendid panorama of the city and its surroundings; here is the belfry with seven bells, which are never rung to avoid that the vibrations of their movement could be dangerous for the stability of the tower. Right from the top of the bell tower it is said that Galileo made his experiments on gravity, dropping some objects and studying their trajectory. The height of the tower, in the countertrend part, measures approximately 58 meters, the weight is 14,453 tons.