Progenitors

The Progenitors were an ancient civilisation that left artefacts and ruins all over the galaxy.

came to Fortuna III to investigate the alien ruins left by the Progenitors.

Ruins
On Fortuna III, they were responsible for building the tower in the centre of Bright Sands, and the ring in the centre of Crescent Falls. They also build the various other ruins of the same material all over Crescent Falls.

Theories
According to Back to Basics part 4, they used nanobots to terraform Fortuna III.

Emmanuel Sullivan believes there is a connection between the green crystal seal at the Crusher Caverns and the terraforming tech of the Progenitors.