It was done, I think, using stencil shadow volumes. I consider it to be an intermediate level 3D programming technique, not too difficult to implement if you know a thing or two about vertex shaders and stencil buffer manipulation.

The basic process is found here:
http://en.wikipedia.org/wiki/Shadow_Volumes

The other possibility was that it was done using perspective shadow maps, but I think that's a little unlikely, considering the shadows had hard edges.

The same technique was used in Doom3.

This technique will supposedly be replaced by perspective shadow maps, however, due to improvements in graphics hardware.

From the sound of it, if you want to do this sort of thing, you're going to have to learn some decent OpenGL or Direct3D. Not a task to be taken lightly, it will probably take a few weeks of daily study to get to the point where you can use this technique, assuming you have a good grounding in a programming language which can use those APIs. Good luck to you! [grin]