The Portal script is one of the more challenging ones I've had to write, because it's very difficult to pick apart a game that manages to excel on all fronts. And now Portal 2 is on the horizon, and it's being hailed by its writers as "one of the finest single-player games [Valve] has ever created". No pressure. Portal is one of the clearest example of the "gameplay before story" adage. It started out as a student project that saw its core mechanic (navigating obstacles and puzzles through the use of two dynamic portals) spun out into an entirely new game. The relatively small development team then had to come up with a fresh story to contextualise that mechanic, and design constraints compelled them to use existing assets from the Source engine. This led to the decision to consolidate Portal into the Half-Life universe, but besides a few vague allusions to Black Mesa and the Combine, the game exists on its own merits. Ironically, Half-Life 2: Episode 2 incorporates more outright references to Portal (and especially Aperture Science) than the other way around. As such, Portal is a game with a story that was written entirely in function of its central gameplay conceit, and that story has been rightfully praised far and wide. Judging by the amount of marketing that surrounds its sequel (not to mention the ongoing ARG that has the fanbase jockeying for position), it seems that it's become one of Valve Software's most prized products. But to be fair, it has ruined cake forever as being just cake.