The defense-in-depth model

No single solution can truly protect against all attack vectors and plug every security hole of a system. By applying a defense in-depth model and layering protective measures, gaps in the security of one layer can be closed by controls on another layer, creating a holistic security posture. The layering of protective measures is much like how back in the middle ages, a king would protect his gold and jewels in a secret treasure room down in the dungeons of his castle:

The first line or layer of defense were the guards on watch towers, keeping an eye out for anything out of the ordinary approaching the castle surroundings. ...

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