Does Crystal Space use Direct3D?
No, Crystal Space works with OpenGL (or software rendering) on all platforms.
Do I need a C++ compiler to use Crystal Space?
Yes and no. To use the basic Crystal Space and Crystal Entity Layer source packages you need a C++ compiler. You will also need a C++ compiler to compile your own project sources. However, you can also use CELstart which is a precompiled application that contains Crystal Space and Crystal Entity Layer. And using CELstart you can make applications with Python or XML without the need for a C++ compiler.
Is there a free C++ compiler available?
How can I make maps for Crystal Space?
One of the best supported solutions for making maps is Blender. Using the powerful blender2crystal script for Blender you can export directly to Crystal Space and Crystal Entity Layer from within Blender.
Included with Crystal Space there is also an exporter script for 3dsMax. This is also well maintained and supports a lot of features in Crystal Space.