Definition
Defense in depth is the principle that security should never rest on a single control. Instead you stack multiple independent layers, so that if one fails, another still stands between the attacker and the asset. The metaphor is a medieval castle: a moat, a wall, a gate, guards, and a keep, each of which an intruder must defeat separately. NIST describes it as applying multiple, overlapping safeguards so that a weakness in one is compensated by the strength of others.
Layers for self-custody
A Bitcoiner practicing defense in depth might combine a hardware signing device (so keys never touch an online machine), a passphrase on top of the seed (so a found backup is not enough), multisig across separate devices and locations (so one compromise is not fatal), an air-gapped signing flow, and physically distributed, durable backups. No single layer is assumed perfect; each exists precisely because the others might fail.
Why one strong lock is not enough
Single-layer security creates a brittle, all-or-nothing failure mode: the moment that one control is bypassed, everything behind it is exposed. Layering converts a catastrophic single point of failure into a series of obstacles, each buying time and raising the attacker's cost. The trade-off is friction, so layers should be chosen deliberately against a real model rather than piled on reflexively, since unused complexity can itself become a weakness.
Defense in depth complements a small attack surface and the access discipline of the principle of least privilege; the surface decides how many doors exist, and the layers decide what waits behind each one.
In Simple Terms
Defense in depth is the principle that security should never rest on a single control. Instead you stack multiple independent layers, so that if one…