I bet you didn't know that CS originally came with a training level, did you? Back when the game was first released at retail, Valve made this training level to get players familiarized with CS's core gameplay mechanics. This is something I wish more developers did. Or at least go the Unreal Tournament route and have bots or something.
This training stage appeared only in non-Steam ("WON" by some, referring to the original online service HL games ran on) versions of Counter-Strike. It's in there as late as 18.104.22.168 (CS 1.5), but it's gone in 1.6. It's not really needed as most people have played it, but it's kind of a bummer it's not in there these days.
Hey, Valve! How about a TF2 training level! Wouldn't hurt anything...
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