Know Before You Go
The fort itself is located in the Berlengas archipelago which can be reached by boat from Peniche, Portugal. Once on Berlenga Grande, another boat will take you to the fortress where you can stay and explore (or you can walk there and back but for 10€ the boat trip over is worth it), or even rent a room at the hostel that is now run out of the fortress.
There is a restaurant and a cafe on the island, both of which are cash only so be prepared. You can also pack a picnic if you’re day-tripping there from Peniche.
On windy days, the trip over can be quite rough. There are opportunities to swim in crystal clear water there so you may want to pack a bathing suit as well.