secret level

In video games, a level (also referred to as a map, mission, stage, course, or round in some older games) is any space available to the player during the course of completion of an objective. Video game levels generally have progressively increasing difficulty to appeal to players with different skill levels. Each level may present new concepts and challenges to keep a player's interest high.
In games with linear progression, levels are areas of a larger world, such as Green Hill Zone. Games may also feature interconnected levels, representing locations. Although the challenge in a game is often to defeat some sort of character, levels are sometimes designed with a movement challenge, such as a jumping puzzle, a form of obstacle course. Players must judge the distance between platforms or ledges and safely jump between them to reach the next area. These puzzles can slow the momentum down for players of fast action games; the first Half-Life's penultimate chapter, "Interloper", featured multiple moving platforms high in the air with enemies firing at the player from all sides.

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