Original: The source code to the Duke Nukem 3D v1 .5 executable , which uses the Build engine , was released as free and open source software under the GPL on April 1 , 2003 .
Simplified: The source code to the Duke Nukem 3D v1 .5 executable , which uses the Build engine , was released as free software under the GPL on April 1 , 2003 .