Portal was an instant classic but it didn't start that way, beginning instead as simple student game before Valve saw the potential and snapped up the team. Over the years the simple (?) idea of space-warping portals was refined more than anything else - most changes seeing tweaks to the aesthetic while the core mechanic remained unchanged.
The Valve News Network seems to know more about the developer than Gaben himself and have put together this excellent making of covering the history of the first Portal: from that student project, to a radically reimagined game in the Source engine using a ton of repurposed Half-life 2 assets. For a while it even used Gordon's gravity gun, for example, instead of the that final sleek white final version.
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