Bridge Command is written using Blitz3D http://www.blitzbasic.com/ (http://www.blitzbasic.com/), and the source files are the .bb files included in the installation. The main source file is bc4.bb. They are text files, so even if you don't have Blitz3d installed, you can look at them with your favourite text editor.
In due course I am planning on re-writing Bridge Command in C++ and an open source 3D engine, both as Blitz3D is a commercial closed source program, and its restriction to DirectX 7 is becoming somewhat limiting. In the mean time I'm developing as far as I can with Blitz3D.