Fortification built in 1270 to provide additional protection on the northern fringe of the city.
Inside there are rare traces of painted decoration that had been lying on several occasions over the centuries.
The Assumption of the Virgin, painted according to tradition by Simone Martini, had given way to the fresco of the same subject to Alexander Casolani commissioned in 1585 and completed in 1589; new interventions and restorations were carried out at the end of the seventeenth century, first by Antonio Nasini (1685) then by his brother Giuseppe Nicola (1699). During the 1944 bombing the latter drafting of plaster collapsed, discovering again the painting of Casolani, of which you can still see some parts.