Since pre-Roman times, Sorrento had a solid defense system ensured by its imposing walls encircling all the town, but it was just in fairly recent history that the walls took their current, partly preserved, aspect. Nowadays, we can just see a few ruins of the most ancient walls under the roadbed of Porta Parsano gate (throughout a grid), thanks to the excavations made in 1921. These blocks are located rearward of the most recent fortification, and were dated to Roman times when they were discovered, thanks to a tablet of Augustan age. Actually, we know that in Roman period Greek walls were still in use, probably built before Samnite invasion of 420 b.C..
Many pieces of potteries and mosaics, glasses, bones and iron objects, where also found, proving that the Greek door opened during Roman times was then filled and bricked up, before being re-opened and widened only in 1745 as the present gate. Thanks to some little but frequent repairs, these walls defended Sorrento during the whole Middle Ages resisting to many attacks.
During the Spanish rule, the town was exposed to the risk of Saracen invasion which devastated lots of coastal towns in southern Italy during the 16th century. That's why in that period Sorrento became a centre of intense activity to guarantee a stronger defense system to the town. The walls' renewal began in 1551, after the damages caused by the war between Angevins and Aragoneses, but only ended in 1561, after that the tragical invasion of Saracens (13th of June 1558 throughout Marina Grande Gate) had induced the citizens to quicken the restoration

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The southern side of the ancient fortification was completely restored and made impregnable by the Spanish engineer Pedro Treviño. The walls were built in rows of tufa stones, almost regular and protected by plaster, with an horizontal string course along the whole length. The Greek-Roman pathway was preserved at most, surrounding the whole town and leaving just a gate at the end of the main decuman and hinge, and on the road towards the two harbors (Marina Grande and Marina Piccola). It's important to remember that these walls are the only example of still standing fortification of Spanish vice-royal period.
The main remaining part is the one on the southern side, from Marina Grande to Parsano Vecchia (the Old Parsano Gate), then to Parsano Nuova (the New Parsano Gate) and finally to the ruins of the Bastions of Piano placed on the eastern side, just upon the deep Vallone dei Mulini. Along this southern side, Treviño restored and improved four still standing Bastions, named with four of the five Patron Saints of Sorrento and joined to each other thanks to reinforced wall tracks called "cortine.