The turtle fortress was built by players from the 'Turtle People' race.

In the Abandoned Gods Game, the Turtle People are considered one of the alien civilizations. Despite being called 'Turtle People', they don't resemble humans at all, nor do they look like the turtles on Blue Star. They simply have hard shells covering their bodies, and their heads can retract into these shells, hence the name Turtle People.

They are veterans in the Abandoned Gods Game.

In fact, the timeline of the Turtle People's existence spans from the Old Era to the Evolution Era. They possess a racial talent that allows them to reduce damage by thousands or even tens of thousands of times when their heads are retracted into their shells.