Tilly, one of the things the rebuilding of Windsor Catle did was train up a new generation of craftsmen, under the guidance of the few older ones. They learnt a lot on the job - finding building techniques that had always puzzled people, but starting from scratch they realised why they'd been used.
A few years ago, a medieval manor (sorry, can't remember its name, but in England) needed a new oak ceiling, the old one being 500 years old and starting to splinter. But they couldn't find any oak trees big enough for the job. Eventually a man knocked at the door saying "I heard you needed oaks..." It turned out he was descended from the family that had supplied the original oak all those years before. As soon as they'd done so, they planted a new forest, knowing it would be needed centuries in the future.