Making plural words possessive can be confusing at times because we so often add an s to a noun to make it plural. All of those s’s can be a little overwhelming.
But the rules are pretty clear on this issue.
To make plural nouns that do not end in s possessive, add ’s.
the mice’s tiny tails
To make plural nouns that end in s possessive, add just the ’.
our zombie fortresses’ weaknesses