I have often compared patents to castles when discussing high stakes invalidation exercises. Some are inherently stronger than others, but given enough resources, none of them are invulnerable. 
Castles, like patents, are often viewed as defensive structures. So it was interesting to hear a narrator on a History Channel episode about great fortifications describe castles as tools of aggression…an inherently offensive and not a defensive weapon. Why would this be so? Answer: because it conforms perfectly to the well-known strategic principle of the offensive defense. The idea behind the offensive defense is to capture something of value to an opponent so that he must attack you where you are prepared to receive him in order to get it back. What better way to cause your opponent consternation and change the nature of the game than to build a castle on land he called his own?
What better way also to understand what it really means to have a patent strategy? It takes a lot of resources to build castles, and there really isn’t a whole lot of point in building one if there isn’t someone out there who might want to attack it. After building such a castle, you can consider building a network of castles that people will want to attack so that if one falls, others still stand, and ideally whoever should wish to attack will decide the better against it.
Image credit: R. Duran