Despite all these years this game still holds up. The way in which Valve designs games in usually very simple. Almost every valve game exists for a reason such as to test a new engine, because they have developed a new gameplay mechanic so good it centralizes the entire game or because new technology has come out that they wish to use. Valve games do a very good job of teaching you how to use the mechanics and systems of the game and portal might be one of the best examples of this. Portal is debatably the best valve game at being a valve game, where you can pitch the game to someone in a single line and promise a grand world to them. All you have to do is tell some that portal is a puzzle game where you can make a portal to manipulate physics and your can already imagine doing so much. Valve games are never too long and oftentimes too short so you never feel like you've seen enough of a mechanic. Portal is an incredibly game with a great mechanics that still holds up.