m1=b1 is a rampart/bulwark/wall/fence protecting m2 from danger/threat m3, physically separating/partitioning b2 from b3 (unordered), of/in structure b4.
The method by which x1 protects x2 is unspecified. It may, for example, physically block x3 from reaching x2, or it may simply give x2 the high ground it needs to protect itself.