In the previous chapters we went back in history to see how the construction of flood defenses developed over time. At the same time, we also looked at the safety and security aspects of those flood defenses. However, over the centuries, water has not only been a threat. It can also be a friend (if it behaves itself). In relation to security, we can learn a lot from the use of water in defensive applications. In Europe, and in Dutch history, water defenses have frequently been used as a defense against foreign invasions and dominance. Flood barriers can not only be used as defense against water, but they can also be used for retaining water. The Dutch invented an ingenious combination of waterworks for such purpose. Understanding of these constructions increases our insight into flood barrier weaknesses, and security- related vulnerabilities.