 A team of masons, labourers, and slaves did the construction work, the Muslim slaves were charged with the hardest labor.In 1521, Philippe Villiers de L'Isle-Adam was elected Grand Master of the Order.Expecting a new Ottoman attack on Rhodes, he continued to strengthen the city's fortifications, work that had begun after the Ottoman invasion of 1480 and the earthquake of 1481, and called upon the Order's knights elsewhere in Europe to come to the island's defence.