I just want to poke about a dll file and see if I understand anything. But I don't even know how to open one. I tried opening one in notepad and it was just a load of gobbledegook.

The dll is compiled from C++ source code, so you would look at that. Should be in the BTS install directory in the CvGameCoreDLL folder, and files like CvPlayer.cpp and CvCity.cpp would be the most interesting. Header files like CvPlayer.h and CvCity.h would give you a general idea of what kind of functions exist for these different objects.

There is a guide how to compile a dll
http://forums.civfanatics.com/showthread.php?t=166933

thanks to both of you :)