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So I was taught that I should use apostrophes when something possesses something else, but I'm not sure about the use of it's in the following situation:
...and they found their way to a castle. It's walls were made of pure gold.
My question is, should I use It's because the apostrophe means that the castle has possession of the wall, or should I not because it's is generally understood to be a concatenation of "it is"? Or perhaps I should use Its' walls?