Salineas is one of the main seven Kingdoms of Etheria. Glimmer stated that "whoever controls Salineas, controls the seas".
At the end of Season 5 when Horde Prime was defeated, it is assumed that Mermista was able to regain control of her kingdom.
This kingdom has a huge sea gate to protect it. The Sea Gate is a magically powered structure that protects the city. According to Adora, the Sea Gate was connected to The Pearl by the First Ones over a millennia ago. The Pearl powers the magical protection provided by the gate.
Princess Mermista inherited the throne from her father upon his retirement. When Salineas is introduced in season one, the Sea Gate's connection to the Peal runestone has been weakening and the gate is slowly falling apart.
Glimmer convinced Queen Angella to send a delegation to Salineas to recruit Mermista and her kingdom to the Princess Alliance. After arriving there, Adora deduced that the decay was caused by an interruption in the connection between the Sea Gate and The Pearl, Salineas' runestone, by reading First Ones' writing on a mural in the throne room. This was one reason the kingdom was crumbling at that time. Resulting of the instability, people fled and the kingdom was understaffed, leaving Princess Mermista as one of the only citizens. She-Ra used the Sword of Protection to heal the Sea Gate.
At the end of "Mer-Mysteries" it is revealed that The Horde had attacked and conquered Salineas using overwhelming numbers while Double Trouble was at Bright Moon creating a distraction. The Horde was then able to take control of the oceanic territory.
Song of the Sea Witch
Salineas is the setting of the children's book, Song of the Sea Witch.
(as Princess' consort)