The csDoom project is based on ZDoom 1.22 which is not released under the GPL, but I incorporated quake world code… QW is released under the GPL and I must release csDoom under the GPL too. I have the right to not release it under the GPL, if I rewrite all QW code! Carmack said: “It is a waste of effort to go and rewrite perfectly good code.” and he was right of course. I asked some questions then and John Carmack convinced me that it’s good to share my work so others can build upon it. The next csDoom version will be released under the GPL!
The current csDoom release, version 0.62, can be downloaded from here.