Sky City is a scrapped location in Serious Sam: The First Encounter.
Sky City is a major location on Sirius, and Mental's current location during the events of TFE. In the original version of TFE, the player's goal was to reach Sirius and kill Mental in Sky City. It is also notable because Sky City is the original location for Gnaars.
Sky City was to be the second-longest area in the game, as it had 9 levels associated with it; Gravitality, Labryinth, Looneymooney, Platformal, Spheretorus, Verticality, GrandFinale3, GrandFinale2, and GrandFinale1. It seems that it's primary gimmick was going to be things like unusual gravity.
The player would reach Sky City after completing Space Crusader.
Sky City was completely removed from TFE, but the idea later resurfaced as Siriusopolis in Serious Sam II. The gravity gimmicks that Sky City was supposed to have resurfaced in Sacred Yards and The Pit, most likely because the unusual gravity was already coded into TFE when Sky City was scrapped.
One from the levels in Sky City was supposed to be used in Test 1 but it was cut.