Survival Mode was a discontinued gamemode that was replaced in favour the Warzone Mode. It is unknown if Survival Mode will be back in game or not.
Survival is a unique mode, played on either the "Survival" map, or the "Spiral " map, in which one to four players have to survive against nine waves of enemy units that run toward the center of the map, where the players' Fortress is located. In addition to the four Pads attached to the fort each map has four Outposts which can be captured by the player to provide additional pads and income.
An endless survival mode can also be played on the map "Last Stand" via the Custom Game option. This functions the same as regular survival, except it's endless and the enemies come out of every nook and cranny. A Last Stand survival match goes until the player is defeated, by having their fortress destroyed.
Many different strategies have been created by people, but they usually rely upon spamming turrets with repair units, or rely on the Ospreys repair ability, as keeping expensive units alive is a must. Artillery has mostly made it a lot easier to win normal maps, but Gorgons keep mechs on their toes. Stasis Mines in combination with artillery, mean massacring creep masses is relatively simple. The "survival" map, is harder than the spiral map, because you have 8 places to guard, rather than just two.