Castles and sorts

[Delthion]

But Salamandastron's very few exits also work to their advantage, and they know of that exit that takes you past the crabs now, and they have a constant water supply.

[Albrithr]

Every egg can be cracked, and every shell: broken.

It doesn't matter how well a castle can be defended, it will eventually fall no matter what under enough enemies assailing it and attempts to gain entrance.

I agree, no fortress can withstand treachery.  That's how Cluny took Redwall, and how Floret fell.