Any single piece should be replaceable without the whole losing its identity.
The Shield is about seams. A system has structural integrity when you can swap, upgrade, or delete any one unit without the rest of the system caring how it was done. The boundaries are real, the dependencies point inward, and no two pieces share mutable state behind your back. Cut a unit out and the shape that remains still tells you what the system is.
Lose the Shield and you get a distributed monolith: a set of modules that look separate but cannot actually move on their own. A “small” change demands edits across several of them at once, because correctness was quietly leaning on a shared assumption no interface ever declared. The pieces have names and folders, but they are welded together.
There is a humble test for integrity that most teams overlook: the revert. You can only cleanly back a change out if that change was small enough to isolate and independent enough to remove. A codebase that ships 800-line commits cannot revert anything; it can only patch forward, forever. A codebase of atomic, single-purpose units can pull a mistake straight out. The revert rate is a quiet proxy for how much structural integrity a system actually has.
Related: pairs with The Buffer (independence from infrastructure) and The Anchor (independence of intent).