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Oh yes, a den is another name for it. I suppose it depends on what sort of creature might inhabit it, and for what purposes. Now, if it was going to be a lion's home or a man's study, then certainly "den" would be most appropriate. For other creatures, "habitat" would work. If some kids made it as a hide-out, then it's probably a fort. I'll have to sneak back there and see what's living in it, and then report back! :-)