Cardinal Gil Albornoz recovered Terni for the papacy in 1355.  However, Terni rebelled in 1357, at the end of Albornoz’ first legation, and the new papal legate, Androin de la Roche requested help from the rectors of the Patrimony and the Marches and from Perugia in order to suppress the rebellion.  The rebuilding of the city walls and the construction of the “cassarum pontis Sancti Antonii” probably followed the rebellion:

1. ✴The cassero was documented for the first time in 1358 in connection with the stipend of its castellan.  This fortress, which was surrounded by a moat that took water from the nearby river, was the most important part of the defensive ring around Terni.
   

2. ✴In 1366, Cardinal Albornoz wrote to the papal rector, Francesco Orsini in relation to a request from the Commune for exemption from taxes in consideration of the cost that it was facing in relation to the new civil defences. 
   

3. ✴In 1372, the Commune responded to a request to send men to help with the construction of the Rocca di Narni by pointing out that this was impossible, partly because Narni was a traditional enemy but also because the available labour was still at work on the defences of Terni (although it seems that the work was by then largely complete).
   

The Cassero stood between Porta Romana and the wooden ponte Sancti Antonii (for both, see the page on City Walls, Gates and Bridges).  When this bridge was destroyed in a flood in 1389, the Commune commissioned a replacement in stone.  The new bridge (1392-5) had defensive towers at each end and was linked by a wall to the Cassero.  Visitors entering the city first crossed the fortified bridge and then twice crossed  the moat as they entered and then left the Cassero, before finally passing through the fortified Porta Romana.

In 1398, Pope Boniface IX appointed his brother, Andrea Tomacelli as papal governor: he recalled the Guelfs from exile and restored the Cassero.  When Boniface IX died in 1404, Andrea Tomacelli refused to hand over the Cassero to Pope Innocent VII.  Papal forces under Ceccolino Michelotti of Perugia invaded Terni and, with the support of the people, tore the Cassero down.  (It is possible that the destruction was less than complete, since payments to the castellan continued).  

In 1436, Cardinal Giovanni Vitelleschi, the legate of Pope Eugenius IV, ordered the rebuilding of the Cassero.  However, in 1442, Eugenius IV agreed to allow the fortress to be demolished.

The church of Santa Maria del Cassero was built on the site in 1546.