This view looks north west across the Golfe de Napoule, with Cannes on the right. (Roaring Girl is the boat to the left, just visible under the tree.)
It is not immediately obvious why the powers that be wanted a fort on this side of the island, looking at the mainland, rather than protecting the approaches from the open sea. But there have been fortifications here for thousands of years, as the many Roman remains testify, illustrating settlement as early as the sixth century BC.
The answer lies in the anchorage, the only secure one in the vicinity of Cannes. For much of Mediterranean history, maritime trade has been relatively short haul and coastal, relying on finding sanctuary easily in the sudden storms and steep seas that haunt the basin.
It seems possible that the few longer-haul ships, carrying diverse cargo from places like Gades (Cadiz) to Ostia (the port of Rome) may have also hugged the coast, and used this as a safe stop. Not always safe however: a 1st century Roman wreck has been found on the rocks to the eastern end of the islands, as well as a 10th century Saracen wreck.
So in addition to be an important prison, the Fort protected the anchorage and His Majesy's trade. This might make us sleep easier at night, but only so long as there is no strong easterly wind!