Multiplayer games for both Mac and Windows?

My girlfriend and I are searching for games where we can play together, local or online multiplayer.
The preference would be RTS games like Age of Empires where we can massacre each other but finding these kind of games has been a bit difficult due to Mac not being so compatible with most of the games.

Do you have any game or website suggestion where we could look for these kind of games?
Also if you have any other gaming suggestion for couples we would love to hear it!

Thanks!