This is an archive of the C++ source files which come with BTS 3.19.
It has 2 changes: It does not include CvTextScreens.cpp which is not used, and a line in CvGameTextMgr.cpp which causes compilation problems (and is never used anyway) was commented out.
See A simple guide to compiling the DLL
CvGameCodeDLL C++ source code files (BTS 3.19) 2016-10-05
CvGameCodeDLL C++ source code files (BTS 3.19)