Sky Fortress Zone is the fourth level in Sonic the Hedgehog 4: Episode II. After beating the heat in Oil Desert Zone, Sonic and Tails take to the skies with the Tornado to take on Eggman's newest airship. The first act involves flying like in Sky Chase Zone, and avoiding Metal Sonic all along the way as he attacks with an onslaught of badniks from his own battle jet: the Metal Carrier. When playing as two players in this act, Tails controls the Tornado and Sonic controls independently, while a single player does it old school with Sonic and the Tornado controlling together. The Tornado is capable of doing a spin attack which is required to break down barriers, but doing so makes Tails dizzy, preventing consecutive uses.
Near the end of the first act, following a platforming section that Sonic must clear alone, Metal Sonic will shoot cannonballs from his ship in a last-ditch attempt to stop Sonic and Tails. These cannonballs can be avoided, and the heroes will eventually reach the Signpost on-board Eggman's ship and end the act, but Sonic can knock the cannonballs back with a Spin Jump. By knocking back 8 cannonballs, Metal Sonic's ship will go down, thus ending the act sooner and awarding the player(s) with the "Metal Sonic Falls" achievement/trophy.
The second and third acts involve traversing the ship itself, much like in Wing Fortress Zone. The second act has the heroes raid the deck, while the third act takes them under the ship's belly at sunset, complete with light beam transportation systems like those seen in Sonic & Knuckles's Death Egg Zone.
||A super-giant flying fortress made to be the final line of defense, Can Sonic fend off the relentless chase from Metal Sonic in a high-altitude battle?
— Description on the Sonic 4 site
"Sky Fortress Zone" was a prototype name for Sonic 2's Wing Fortress Zone, as evidenced by the level select screen in some of the prototype builds.
- Act 1 - The Tornado Takes Off!
- Act 2 - Infiltrating the Sky Fortress
- Act 3 - Mile-High Mayhem
- Boss - Showdown in the Skies