You need the cynical law of organizational or personal collapse: every defensive system is defeated by the insider, the double-cross, or the self-sabotage that no perimeter can stop.

Dagenham's law is the novel's thesis on why every fortress fails: all armor faces outward, but the fatal wound always comes from inside. Vorga passed Foyle by through a human decision; the Outer Satellites' plot dies through their own fanaticism; Presteign is undone by his own daughter's hatred. Bester's hard rule: the breach is never the wall, it's the man guarding the wall.