This is an old and stilted one. It needs alot of work for me to be happy with it. I include not for it's merit technically, but because I like the message it was trying to convey.
Oh build ye your walls of solid stone,
and forge them into a fortress,
all your own.
Build them well and build them true,
brace them with beams of solid yew,
it will do no good.
They might last a year or even more,
but someone will find the secret door,
and come stealing in.
They'll take the keystone of it all,
and slink away as it tumbles down,
dust and ruins again.
But whatever you do don't ask,
merely rebuild as is your task,
there is no reason.