Sabine Baring-Gould - Cliff Castles and Cave Dwellings of Europe (1911)
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Sometimes I think this blog should just repost all of Baring-Gould and Andrew Lang and be done with it. Certainly I never expected to find something like this which covers pretty much what it says from castles on the sides of cliffs to subterranean churches to cave oracles to robber's dens. One early chapter about "wild" people is called "Modern Troglodytes". As usual with Baring-Gould he can't resist any kind of odd story from anywhere in history whether it's some strange classical legend, a peculiar antiquarian find or an excerpt from Froissart (though his focus on Europe means he skips the incredible Mogao Caves along the Silk Road).