When you think about self defense, you need to approach it from a military or police mindset. That means, your defenses need to be in place before the need occurs. If you think about the modern military or even medieval military, the castle, the walls, moats, forts, bunkers, etc. were all put into place before any attack occurred.

They put these measures in place in anticipation of a possible attack. If they waited for the first shots to be fired and the approach of the enemy, it would be way too late. Same with police officers – their bullet proof vests are designed to defend them from gun fire. However, the officer must put the vest on before he or she goes out on patrol. If they wait until a gun is drawn on them, again it is way too late. This is the mindset that you need to have when you think about your own defense.