We got this question last night while playing with the barn and the hill, and other expansions, naturally. We weren't sure whether one can put a barn on a corner that where one of the four tiles features a hill and thus higher than the other three. My friend and I agreed that it's okay to put the barn there, even when it seems like aftermath of a landslide. I didn't get an answer in CAR (maybe there is one, but I simply skipped it.). Do all the four tiles have to be on the same level to build a barn or does it not matter to have a tilted building?
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