There are no impenetrable defenses. The Maginot Line was considered an impenetrable defense and it proved to be a liability, not an asset, as the French built their entire defensive strategy around a single idea. The best defense is to live in peace where you are engaged socially and economically with your neighbors. Short of that it is foolish to rely solely on the idea that there are impenetrable defenses.
UN: Gaza could be 'uninhabitable' by 2020 if trends continue (AP)
from the article: Iron Dome is not enough to protect Israeli citizens