Eduke32 comprises the gameplay engine released in 2003 by 3DRealms under the GPL, and BUILD, the 2.5D graphics engine developed by Ken Silverman. Sources for BUILD are available, it's not open source, but has a crippled license, there is also this FAQ on its page:
Clearly noone explained him the power of free software, this engine could be much more popular if under GPL like the id's ones rather the its crippled license.Why did you write a custom license instead of using GPL (GNU's General Public License)?
Short answer: to satisfy my ego Long answer: I know GPL is an industry standard, but I felt that it was way too long and boring for anyone to read seriously. I chose to write a nice short license that everybody could understand. This way, people unfamiliar with GPL would understand their rights.