Not only does it contain Shiki's army, it also contains people who have feathers on their arms that enable flight. The island is also home to the rare IQ plant, in which Indigo used as a basis for his SIQ drugs for creating the mutant army, as well as the Daft Green.
When it was under the Fuwa Fuwa No Mi's influence, Merveille comprised of several floating islands suspended in the air, each with its own flora and ecosystem. It even housed several bodies of water, notably the falls.
Many of Merveille's human citizens have been mutated to a small degree, likely due to exposure to SIQ, which has caused them to grow feathers underneath their arms and give them the ability to fly.
The island was turned into a floating sky fortress with Shiki's levitating abilities, and an ancient Japanese-stylized palace in the center of it. After a grueling battle between Shiki and Monkey D. Luffy, when the latter destroyed by Luffy's attack. Shiki's flying powers over the little islands around Merveille was deactivated, and came crashing down into the sea where the land mass contracted, trapping the mutated beasts within, freeing them from Shiki's control but still remaining secluded from the rest of the world.
