Not to be confused with the former fortress of the same name, only the foundations of the Baluardo di San Paolo (Bastion of San Paolo) were laid in 1585, under the supervision of architect Giovanni Battista Aleotti (aka l’Argenta, 1546 - Ferrara 1636) and Lieutenant Cornelio Bentivoglio.

A long curtain wall joined the pentagonal pontifical fortress to the defensive structures of the Porta di San Paolo: this stretch of wall, built in the seventeenth century and reworked in the nineteenth century, replaced the earlier medieval defences which lay further back; these were demolished when the papal fortress was built at the beginning of the seventeenth century. The curtain wall was still intact in the early 1930s: at that time, as part of an industrial plan which included extending urban development towards the south, a long segment of the southern walls adjacent to the former fortress was demolished to make way for the construction of the new aqueduct tower and other infrastructure in the Piazza d'Armi area.

The expansion of the Rione Giardino quarter towards the north-west industrial zone and the anthropised one to the south/south-west (along and just before the trajectory of Via Bologna) was completed in the 1950s, resulting in all visible traces of the walls being lost for a stretch of around 450 metres - the largest breach in the entire city boundary. Another hole was torn open in 1967 where the current Via Kennedy begins, while the embankment of the residual stretch of wall to the west was further thinned in 1981, during the works to build the new bus station.