When the Knights of St. John were granted Malta as a fief by Emperor Charles V in 1530, they sent a commission to examine the islands and prepare an informative report. This fact-finding mission was disappointed by the islands' lack of greenery and the dearth of fortifications which would, in due time, thwart their Ottoman enemies. The memories of Rhodes was still in their minds. The Grand Harbour was Malta's only commendable asset. It was there that the Order's fleet would take shelter and be repaired after the various attacks from Ottoman galleys.

On their arrival, the Knights embarked on a building spree in order to protect themselves and the Maltese from the imminent Turkish invasion. At that time the defenses of the islands were limited to the bastions encircling the old capital of Mdina and Rabat in Gozo. The Castello al Mare at Birgu was the only fortification which guarded the harbour area, an area which was of vital importance to the Knights.