Andrea Pisano bronze bell; the tower was plastered and its spire added in 1862. The Museum of Sacred Art is in the cathedral's crypt. In the late Twentieth century the church was closed in order to undergo a restoration that will last until 2013. During the restoration, the floor tiles made of white marble and the Nineteenth century altar were removed. Church of San Francesco The Church of San Francesco has a gabled façade with smooth stonework; the door in its centre is surmounted by a rose window and flanked by two side oculi. Its wood-ceilinged nave is divided into seven bays, flanked by seven side chapels; the chapels and the presbytery were added during the 16th century. The chapels all have simple cross vaulting (meeting in a hanging pendant), except for the Crucifix Chapel. The stone choir replaced the original wooden choir, which was demolished at the beginning of the 20th century. The church has a marble font and artworks in the chapels, including a retablo by Antioco Mainas. The monastery, with the San Francesco Cloister, was annexed to the church during the 16th century. Church of Santa Maria di Valverde The Church of Santa Maria di Valverde is outside the town walls and nearly contemporary with the cathedral, with similar style and structure. Like the cathedral, it has a typical Romanesque style with Gothic features. The church's façade underwent conservation and restoration until the twentieth century. Built of pink trachyte ashlar, it is divided into two parts by a horizontal, moulded cornice. Its nave originally had a lower, wooden roof; it ends in a large, square, cross-vaulted presbytery with four pendants. The central pendant, larger than the others, depicts the Madonna and child. Medieval fortifications Pisan walls surround the city's historic centre, following its irregular outline and using its incline with the Castle of Salvaterra. Although urban expansion led to the incorporation of several stretches of wall into private homes, the remnants retain the features of medieval military fortifications: blind façades of mixed stones arranged in horizontal courses to create an uneven mass, providing resistance to attack. With 23 towers, the walls could be passed through four gateways: Porta Maestra, Porta Castello, Porta Sant Antonio and Porta Nuova. Town Hall Town Hall is at the heart of the historic town centre relative to the medieval walls. According to plans by civil engineer Antonio Cao Pinna, it was built