Why was a concentric castle much easier to defend?

Well there is are two walls and in between those walls is called the 'deathhole' because your certain to die if your trapped between there, because there will be hot oil or tar being poured on you also fire arrows and arrows will get shot at you. Also there are many towers and turrets which have murder holes in them allowing them to shoot at the enemies with fire and normal arrows they can also catapult rocks and other items at you.