How to Design Successful Game Levels

Levels are a term familiar to every one of us gamers. Since the first time we played a game, we've been in contact with levels. "Which level are you on?" "Have you completed the game yet?" These familiar catchphrases become the starting point for discussing a game with friends. So what exactly is a level? The definition of a level clearly differs across different types of games. A scene in an action game, a map in a real-time strategy game, a race track in a racing game, a dungeon in an American RPG or a maze in a Japanese or Chinese RPG - all these fall under the category of levels. So, what are the criteria for a fun level?