Thinking of a patent as a fortress to defend an invention could be setting yourself up to lose it.

Patents as static defenses can be defeated, just as assuredly as kings of old could defeat any castle by using enough ingenuity or sheer raw force.

To win in patent competition today, it becomes important to make your patent defenses dynamic.  This means that you file patent applications and secure patents with the next filings in mind.  If a patent you have today is successfully challenged tomorrow, then you have new patent documents on improvements, additions, continuations, or other patentable aspects of your product and service solutions, to keep your defense alive.  It also means leveraging all the other advantages of your business, like branding and distribution networks, so that you can still succeed in business without any patents at all.

If you can succeed in business without a key patent that you hold, then that patent can serve to strengthen your business instead of serving as a crutch to shore up weakness against better positioned competitors.  If you cannot succeed without a patent, then you can understand why, and redouble your defenses accordingly.  In any case, never leave an important patent to stand on its own like a castle of old.  If someone wants in badly enough, he will likely find a way.