This making of vid remembers when Portal was set in Half-Life 2's City 17...

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