Games with open or free-roaming worlds typically lack the invisible walls and loading screens common in linear level designs. Generally, open-world games still enforce many restrictions in the game environment, either because of absolute technical limitations or in-game limitations imposed by a game's linearity. Examples of high level of autonomy in computer games can be found in massively multiplayer online role-playing games (MMORPG) or in single-player games adhering to the open world concept such as the Fallout series. The main appeal of open-world gameplay is that they provide a simulated reality and allow players to develop their character and its behavior in the direction of their choosing. In these cases, there is often no concrete goal or end to the game.