The least preserved of Porto Ercole's three Spanish-era forts, Forte Santa Caterina is privately owned.
From Forte Fillipo you can admire the stone and whitewashed plaster walls of this pentagonal fort, which is protected from the sea by another reinforced stone wall.
The wall hanging, slits and windows that can be seen on the outer wall were once used to hold gunboats.
Porto Ercole has some of the most incredible seaside walking paths and even if you never even glimpse this fort, it's still worth the effort just to take in the beautiful sea and gorgeous vegetation of the coast.
And I guess if you are a bit of a history nut, you can see how close you can actually get to the fortress. The owners are used to it and don't really mind the company as long you keep to yourself.