Defense in depth is pretty simple, All it means is not concentrating all the defenses in one line/strongpoint/whatever. Instead the main defenses need to be built a good distance (about the range of a Klink-hammer at least) from whatever they're defending (like a major resource center or staging area). Then, multiple smaller lines are needed behind the main line, and (assuming it can be put there without getting the engineers killed), a lesser line is put in front of the main line. The rearward lines should contain factories.