Maybe it's a drought after that night until spring? /s

That said, they really can just sneak into the island at night and climb up. Ninja girl did it. Tsukasa and spear dood can do it too, and the two of them can wipe them out right now. Guns only mean anything if you have normal human abilities. Killing lions with bare hands, dodging crossbows, well, guns mean shit unless you charge at a line shooting at you.