The Castalians are the collective human derived ethnic groups of the people who live on the planet Castalia. There are 2 main groups of Castalians, the air breathers who are the minority, and the water breathers, who are the majority.
The Castalians first came into being when humanity joined the Systems Commonwealth and genetic engineering became common place. They were engineered to survive on a world that had an extremely rich ocean ecology and little usable landmass.
Some time after the Fall, the Castalians were invaded, conquered and enslaved by the Volsung Pride for 200 years, before they succeeded in defeating them. The Castalian Republic was soon thereafter established.
By 300 AFC, the remaining High Guard ship, the Andromeda Ascendant, petitioned Castalia's help in restoring the Commonwealth. When President Lee was assassinated, unfortunately everything pointed to the ship's only Nietzschean crewmate, Tyr Anasazi. It became apparent, however, that Chancellor Chandos was guilty. Castalia soon afterwards took its place as the second world to join the new Commnwealth.
Being water-breathers, the Castalians require reverse scuba gear to breathe while out of the water.