The "Sky Spire" is the location where Pearl, Amethyst, and Steven went to look for the Heaven Beetle in "Giant Woman".
About
The Sky Spire was where the Heaven Beetle resided. Steven, Pearl, and Amethyst travelled here and collected the beetle after a confrontation with Big Bird. The Big Bird had swallowed the Heaven Beetle some time prior to the arrival of the three, and Steven retrieved it while in the Big Bird's stomach. Steven was rescued from the monster's belly by Opal.
In "Warp Tour" it is revealed that Steven Jr. and his unnamed mate had offspring, including Steven The Third. They all reside here.