There's so much knowledge that we have lost about medieval crafting - but some of it has been re-discovered, or has been revived. Watching these old crafts come to life again is something utterly fascinating, and I always find it amazing how much time and effort goes into something that, at first glance, seems quite simple. Such as a rafter holding up a roof.
A Swedish museum has posted this video on Youtube, and if you have twenty minutes to spare, it's amazing: