Game design owes a lot to artists - it is these people that craft the whole appearance of the game, and often some of the levels too. As you'll see here, Half-Life owes more than most to the creative talent who worked for Valve at the time.
Imagine yourself in the position of the artist - could you produce this level of not only artistic ability, but creativity? It may look easy once you see the final product, but the challenge is in getting there. Game design isn't all about programmers and people with computers - it involves more pencil-and-paper drawing than most of the players would have you believe.