It may take you a moment to fully realize the genius of this idea...

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Hire SUPERVILLAINS to test the defenses!

There's no more reliable test than that!

I can send a message across all of Equestria: "Canterlot Castle has come under increased attack from powerful adversaries lately and we want to provide the best possible defense against them. Any powerful adversaries will be rewarded handsomely for providing authentic attack scenarios on our castle and give us feedback about what parts of our defense could use improving. - Banding together for concerted attacks is welcome."

I can picture it:

A cosmic death ray (from SPACE, you know, where no one is standing guard!!!) melts the castle into a pile of glass.

Regal sisters: "Excellent! We did not expect that. Next time we will be better prepared. Thank you for your services. ... Can we pay you in glass beads?"

Pinkie scriptum: Every factory-new castle needs a break-in phase or it will never develop its full power.