When everything works together, no single solution has to bear the total responsibility of deterring an attacker. And the required strength of each individual countermeasure depends on the others. A $5 door lock may be sufficient given the fence and guards. A $50 door lock may be wasteful given the open window nearby. Poisoned punji sticks might be superfluous given the whirling steel blades.
(Yes, I'm reading this book slow, but that's because I only read it during my anti rsi break, which is 7 minutes every 45 minutes.)