Defense in Depth

Introduction

Have you ever watched a movie where soldiers are storming a castle they may have to pass over a moat or rampart then they would need to find a way through or over the curtain wall all the while the invaders would be under attack from soldiers in the battlements and even if they got that far they still need to get into the interior of the castle which involves more walls towers and soldiers castles were so difficult to attack because they had several layers of security compromising one layer alone does not win the day?