The Byzantine walls of Rhodes (7th ? 13th c.) split the town into three enclaves: the acropolis (later known as the Grand Masters? Palace), the Collachio (upper town) and the borgo (lower town). The curtain and its towers were protected by a fausse-braye and dry moat. Parts of these walls survive to this day incorporated into later structures, particularly in the Collachio. Under the Knights Hospitaller (1309-1522) the town spread, so that the walls came to contain an area larger by two-fifths in comparison to Byzantine times.
The defences of Rhodes reflect the evolution of fortification resulting from changes in siege warfare brought about by the increasing use of gunpowder. By the mid-15th century the role of artillery was decisive and mining more effective than ever. With successive adaptations, the Knights tried to achieve effective flanking fire at every point, solidity and massed firepower to repel attack. Innovations respected, as far as possible, priorities of earlier times- e.g. capability to isolate advance defences from the main line of the fortifications; directing assaults into traps; or the heavy fortification of gates. Modifications were carried out gradually, with limited resources and under constant threat of attack. Thus, sections in relatively sheltered positions preserved their medieval aspect; such are the north and sea walls, which were protected by the Grand Masters? Palace and the fortified moles.
Until about 1430 the Hospitaller defences were flanked by independent square towers open at the back. Their base was protected by a fausse-braye and dry moat. There were at least seven gates and the entrance of the harbour was guarded by the tall Tower of Naillac. The first round towers were constructed after the failure of a Mamluk siege in 1444. Grand masters Jacques de Milly (1454-61) and Pere Ramon Zacosta (1461-7) systematically reinforced towers and landward gates, while Giambattista degli Orsini (1467-76) improved the moat and sea wall. His successor, Pierre d?Aubusson (1476-1503) successfully resisted the great Ottoman siege of 1480. Afterwards, he reduced the number of land gates to four, strengthening them further; he built massive revetted earthworks to shield the more exposed sections of the walls; and widened the moat. Later the Hospitallers sealed another land gate and added to the fortifications west of the Grand Masters? Palace (Amboise Gate). Grand master Fabrizio del Carretto (1513-21) trebled the thickness of the land wall by adding packed earth and masonry on the inside; thus the south and west walls became a continuous gun platform able to hold back massive enemy attacks.
The siege of 1522 lasted six months and led to the surrender of Rhodes to the Ottoman Turks when the defenders' gunpowder was exhausted, and to the departure of the Knights of St. John from the Dodecanese. It caused considerable damage to the fortifications, particularly weaker structures such as battlements and the fausse-braye or parts facing enemy gun batteries. When they took the town, the Turks carried out extensive repairs on the defences but made no substantial alterations.