During the 15th and the early part of the 16th centuries, the shores of Malta were subjected to frequent incursions by Barbary corsairs and by the dreaded Ottoman Turks. Soon after their arrival in Malta in 1530, the Knights set themselves the task of strengthening the weak defences of the island. They reinforced the strongholds of St. Elmo and St. Angelo, as well as the few towers and defensive walls protecting Birgu and its vicinity. After the siege of 1565, Grand Master La Vallette embarked on the
construction of a new walled city - Valletta. Plans were drawn up by the local engineer Gerolamo Cassar.
These first plans were modified on the advice of Gabrio Serbelloni, a military expert, and early in 1566 a final plan was evolved and approved by the Order's council.
The outlines of the fortifications were marked, and the building of both the walls and of the city started in March 1566. A force of 8,000 workers was recruited - mostly able-bodied Maltese men, slaves and some foreigners.
Construction work began at an intensive pace. Ditches and moats were dug and massive walls erected, so much so that Grand Master del Monte, who died in 1572, lived to see the completion of the defensive belt around the site of the new city. This included 30 bastions, varying in height between 18 and 50m, several deep ditches and two gates, viz. Porta S. Giorgio (now City Gate) and the Marsamxett Gate.
Under the rule of Grand Master Hugh de Verdale, a series of bastions, curtains, ravelins, counterguards and cavalier towers were completed and fifty guns were placed in position.
In 1635, Pietro Paolo Floriani and, later in 1687, Karl von Grunenberg, both famous military engineers, were called upon to plan the extension of the lines to cover the suburb of Floriana and the harbour approaches. Their works included St. Francis and Notre Dame ravelins, St. Philip's, St. Saviour's and Sa Maison Bastions, and the vast bulwark commonly known as the Horn and the Crown Works which flank the Grand Harbour outside Port des Bombes.
Two symmetrical Cavalier Towers were constructed on either side of the main gate, to serve as observation posts dominating the whole defensive system.
In the 19th century, the British made further additions and modifications to the existing fortifications. These extensive works rendered Valletta an impregnable citadel.
