Sharky Extreme has an interview with Ken Birdwell and Josh Weier of Valve Software:

SE - So how far done is all of Half-Life 2?
Ken - We've got all the game laid out. We've got about half of it in gameplay testing. Half-Life 2 is designed along a pipeline scheme: we build the maps, we lay out the geometry, the physics elements which are important to the gameplay and then we go thru play testing until we have a whole area of the game that's fun, that runs, that does what we want, plays how we want it. Then it will go on to the finish art team that will lay down all the textures, put the lights in, and put the detailed props in. It's a very time consuming process. To make a game this rich, and a world this natural, we want to make sure first that it's fun and then go through and make it beautiful.

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