You just download Enemy Territory (a stand-alone free game), the 2.6 ET patch, and TCE, done.

I don't understand why this game is so addictive.

I guess it's because the weapons are realistically deadly but the sight system evens out the playing field to thinkers. Rather than being up against people that can headshot you in .4 seconds jumping around the corner.

And I don't think there has ever been a better shotgun implementation ever.
You gotta play the game how you'd do it in real life. It can be tough to quickly process teammates versus enemies. You don't just run around with a magic cross hair giving you perfect aim no matter what.

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