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According to Wikipedia:
中つ国（なかつくに）とは中間の国、中央の国を意味し、「つ」は現代語の「の」に相当する。中津国、中国とも書く。
I looked for 津 in a dictionary but it seems to be an 当て字 so I did not get new information. But I can't get why の was not used in place of つ (since つ and の seems to be equivalent here). Indeed, の is a very old word, why つ has been preferred? What are the nuances that つ conveys and の does not?
つ is just an older version of the particle の. Its use here, assuming that you are referring to Tolkien's Middle-earth, is literary and adds an older and more mystical flavor in the same way that using a lot of older words of English origin might.
