Monterreal Fortress is located at the end of Bayonne promenade, on the peninsula of Monte Boi. You can go inside by paying 1 €. The interior is huge, so you can quietly spend a couple of hours walking around. The views of the coast from its towers are great, this alone is worth going in for. From inside, you can walk through the Monte Boi, to see the outside of the fortress. However, be careful not to miss the first turn as we did, as you then end up back at the entrance and have to walk all the way back!