The walls encircling the old town remain intact, even as the city expanded and modernized, unusual for cities in the region. Initially built as a defensive rampart, once the walls lost their military importance they became a pedestrian promenade, the Passeggiata delle Mura Urbane, a street atop the walls linking the bastions. It passes through the Bastions of Santa Croce, San Frediano, San Martino, San Pietro/Battisti, San Salvatore, La Libertà/Cairoli, San Regolo, San Colombano, Santa Maria, San Paolino/Catalani, and San Donato; and over the gates (Porte): San Donato, Santa Maria, San Jacopo, Elisa, San Pietro, and Sant'Anna. Each of the four principal sides of the structure is lined with a different tree species than the others.