The "Sky Spire" is the location where Pearl, Amethyst, and Steven went to look for the Heaven Beetle in the episode "Giant Woman".
About
The Sky Spire was where the Heaven Beetle resided. Steven, Pearl, and Amethyst traveled here to collect the beetle, succeeding only after a confrontation with a 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., his mate and offspring (including Steven The Third) all reside here.
Schematics for the Sky Spire are later seen on Moon Base computer display during "It Could've Been Great".