Metal Gear: Skyfall, released outside of Japan as Metal Gear: Altitude due to copyright reasons, is a non-canon Metal Gear spin-off made by Dreams Inc. for the Wii U and PS3/PS4.
In the year 2020, a strangely younger Solid Snake is dropped off at a giant skyhigh facility floating above the pacific ocean with the mission of stopping the supposed fabrication of a new Metal Gear.
See full story at: 2020 Skyfall Incident.
The game goes back to the original roots of Metal Gear, being a top-down overhead game, though just like in Metal Gear Solid you can switch to first person to aim better. The game is still Stealth/Action and you must sneak around to avoid enemies.
Just like other Metal Gear games, there are 3 phases, if you're spotted an enemy you'll go into the "Alert" phase where enemies will rush in trying to kill you, if you avoid them for enough time you'll go into the "Caution" phase where the enemies will scout the area to look for you and if they don't spot you for enough time you'll go back to the "Calm" phase.