The files that contains all the information about the \"environments\", that is cities, buildings, and so on, are in the files called \"world\" in the zips that you can find in /art/world/
Those files respect an XML syntax, and, well, I don\' t think they are so hard to understand.
Anyway, if you give a look at Crystal Space documentation you will find some good reference on how those files should be built.
Crystal Space documentation is available at their main site,
http://www.crystalspace3d.orgUsually, for building a model, everybody use a modelling program, ie: Blender, 3d Studio Max, Maya, Milkshape, etc.etc.
Most of them have an exporter to Crystal Space: the exporter makes possible to get a 3d model in \"XML format\".
It is also true that somebody could enter the information manually. For example, flarge, a Crystal Space world, is made manually (at least, from what I know).
Other information?
Ary