Returning to Palermo, Fazello undertook to teach philosophy and at the same time kept up his religious exercises. He so devoted himself to his studies that eventually he gave up all but one meal a day and reduced the number of hours he slept each night. His history of Sicily, De rebus siculis decades duae (Palermo, 1558), which was his only publication, included material on the ancient history and antiquities of Sicily, showing an immense personal knowledge of topography that allowed him to identify, on the basis of ancient authors, many of the major sites of Sicily.
Returning to Palermo, Fazello took on teaching philosophy while continuing his spiritual practices. He became so intensely focused on his studies that he eventually limited himself to one meal a day and reduced the number of hours he slept each night. His only published work, De rebus siculis decades duae (Palermo, 1558), covered ancient Sicily's history and antiquities, showcasing his extensive personal knowledge of the island's geography.