Built on the side of a rock, these gardens are known for their cactus collection. They were begun in the 19th century by Prince Albert I, who was a naturalist and scientist. He spotted some succulents growing in the palace gardens and created this garden from them. The garden was later opened to the public in 1933. You can also explore the grottoes here, as well as the Musée d'Anthropologie Préhistorique. The view of the principality is splendid.