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. To that point, the clearing around a castle is a form of defense as well. By keeping an open stretch of land around the castle, there were no easy hiding places for an attacker to use. The second layer of defense was the moat surrounding the castle, often filled with hungry crocodiles as movies would like to have you believe. If a thief made it...