Tremulous is an Open Source game based on the Quake 3 engine. It is a first-person shooter with some interesting RTS elements. You can download the complete game on the website, both Windows and linux versions are available. The OS/X version can be found on tjw's website.
My first experience with mapping were some experiments for the original Quake. At the time, I was using the Qoole editor. I never got to the point were I could release something finished and I didn't create any new maps until I started playing Tremulous. Some mysterious force compelled me to create a map for this game.
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