Why won't my map compile?

'csg.exe' is the program used to turn your .map files into playable .cmf files, but sometimes it refuses to produce the compiled file. Usually it crashes when it can't compile something.

This is, as far as I have seen, always because of a brush that it cannot handle. Shapes that should be avoided (even if the map compiles) are:

• Flat brushes - All of the brush's vertices lie on a single plane in 3D space (the brush has no volume)
• Bendy faces - Where a face is not flat (ie all of the face's vertices don't lie on the same plane in 3D space). This can be detected using Hammer's 'Check map for problems' feature, although its efforts at 'fixing' the problem are somewhat suspect. You can avoid this problem by ensuring that any oddly angled faces (ones not aligned to the axes) only have three vertices, since any three separate vertices always lie on a plane in 3D space.
• Concave brushes - When any two faces on a brush can 'see' each other. PacMan is a 2D example, because the two edges making his 'mouth' can 'see' each other.
• Multiple face planes - No brush should have two faces on the same plane in 3D space.