Imagine if a medieval castle was an animal. You are imagining a tortoise. They have evolved defenses in every imaginable way, and as a byproduct, they have an incredibly extended lifespan. A lot of it also has to do with their diet, and the fact that they don't use a lot of energy. Their bodies don't experience a lot of wear and tear, because evolution has made them into walking tanks. It's kind of funny how it works, too, because if it weren't for the fact that they're so overbuilt for protection, they would probably be able to move faster. If they could move faster, they would put more wear and tear on their bodies and die younger, but they can't, so they don't.
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