In the Land of Leadale, Vol. 5 (light novel)

One day, Cayna receives an urgent correspondence from Shining Saber, a
fellow Leadale player also transported to this new world. It turns out that a
horde of monsters is headed for the royal capital, and he needs Cayna to lend
her mighty powers to stop the beasts. But when a Guardian Tower in the form of a
giant tortoise appears in Cayna’s absence, will Exis and Quolke have what
it takes to handle it themselves?!