Apventure to Atlantis is the fourth game in a series that began with Dungeon Campaign, one of the earliest CRPGs, in 1978. Wilderness Campaign (1979) offered a similar approach to gameplay but in an overland map rather than a dungeon. Odyssey: The Compleat Apventure (1980) fused the two approaches by offering both a wilderness map and a dungeon map (along with a new sea map) in the same game. (Links are to my posts on the three games.) The games were created and programmed by Robert Clardy and published by Synergistic Software.
Apventure is a direct sequel to Odyssey, beginning on the island, and in the castle, conquered by the player at the end of that game. The back story does a good job elaborating on the story of Odyssey without contradicting it. In Odyssey, we learned that the game world, the Sargalo Sea and its islands, was separated from the rest of the world by a wizard called the High One. He figured if he could wall off the Sargalo Sea from its enemies, he could use the years to better develop the martial capacities of his citizens and be ready to contend with exterior threats when the wall came down. Unfortunately, he ended up walling some evil types inside as well, and they eventually overthrew him. The quest in Odyssey was about usurping the usurper.