Secret level

A level, map, area, stage, world, rack, board, zone, or phase in a video game is the total space available to the player during the course of completing a discrete objective. The term "level" can also refer to difficulty level, as in a degree of difficulty.

The use of levels in video games dates back to Namco's shoot 'em up Galaxian, released in 1979 during the golden age of video arcade games. The term level used during this era of arcade video games represented a difficulty phase or defined section of a given game, as in Galaga (stage 2) or Dungeon.

Another early example of the term "level" is from early role-playing games, where it referred to level of a dungeon—the setting most such games were played in. Players would begin at the bottom (level 1), and proceed through increasingly numbered levels (of increasing difficulty) until they reached their freedom at the top, or they would start at the top (which would also be level 1), and proceed through increasingly numbered (and difficult) levels until they reached the treasure at the bottom.

In games with linear progression, levels are areas of a larger world. Games may also feature interconnected levels, representing locations.

Each level usually has an associated objective, which may be as simple as walking from point A to point B. When the objective is completed, the player usually moves on to the next level. If the player fails, they must usually try the same level again or perhaps return to the very start of the game. In games with multiple human players, the level may simply end once a limit in points or time has been reached. Not all games order the levels in a linear sequence; some games allow the player to revisit levels or complete them in any order, sometimes with an overworld in which the player can transition from one level to another. An example of this is The Legend of Zelda.

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Wikipedia