What is the ruling on women praying in an upper prayer area that is in the middle of the mosque, while men pray below and behind them?

There is no objection to women praying in the manner described in the question, as they are separate from the men's rows and there is no mixing. This is the reason for the permissibility of women standing behind men in prayer rows. This is permissible if the prayer area is behind the Imam. However, if the worshiper is entirely in front of the Imam, then the prayer of the congregants is invalid due to their being ahead of the Imam. Otherwise, only the prayer of the advanced row is invalid, and this is the view of the majority. Malik, however, held that the prayer of the congregant is valid even if they are in front of the Imam. There is a third opinion, preferred by Sheikh al-Islam Ibn Taymiyyah, which states the validity of prayer with an excuse, such as crowding that makes it impossible to pray Friday prayers or funeral prayers except in front of the Imam.