Complex sentence: For that reason, game data such as levels, graphics, sound effects and music are stored separately from the game engine, in "WAD files", this allowed players to make their own without any modification to the engine itself.
Split into simpler sentences: For that reason, game data such as levels, graphics, sound effects and music are stored separately from the game engine, in "WAD files". This allowed players to make their own data without any modification to the engine itself.