Video games with open or free-roaming worlds typically lack the invisible walls and loading screens common in linear level designs.
In recent years game designers have attempted to encourage emergent play by providing players with tools to expand games through their own actions.
Examples include in-game web browsers in EVE Online and The Matrix Online; XML integration tools and programming languages in Second Life; shifting exchange rates in Entropia Universe; and the complex object-and-grammar system used to solve puzzles in Scribblenauts.
The popular Assassin's Creed series, which began in 2007, allows players to explore historic open-world settings.
These include the Holy Land during the Crusades, Renaissance Rome, New England during the American Revolution, the Caribbean during The Golden Age of Piracy, Paris during the French Revolution, and London at the height of the Industrial Revolution. R.: Shadow of Chernobyl was developed by GSC Game World in 2007, followed by two other games, a prequel and a sequel.
Procedural generation also made it possible for the developers of Elite, David Braben and Ian Bell, to fit the entire game—including thousands of planets, dozens of trade commodities, multiple ship types and a plausible economic system—into less than 22 kilobytes of memory.