Rise of Nations is easily one of the most underrated Strategy games of all-time. It's about as close to the mid-point between Age of Empires and Civilization that you can possibly get mechanically, though never achieved quite the same level of admiration those two franchises did. That said, it did manage to accrue a smaller, but just as dedicated fan base, and with its release on the Windows Store, it is currently on sale for the meager sum of $5.
Rise of Nations: Extended Edition, which came out back in 2014, includes the Thrones and Patriots expansion as well as a few graphical upgrades. The same deal can also be had on Steam, which is significant because the Steam and Windows Store versions have cross-platform compatibility, so no matter which one you call home, you can get the same game, for the same great price.
The sale will last until next week on September 28th.