The Age of Empires series is continuing on in a 'freemium' format for download and at retail, I'd speculate that the retail version would contain paid content to justify charging for the basic version of the game you could download. It is also unclear how much content will be free and how much is paid at this stage, but if the game is as fun as previous iterations purchasing content to expand the game is likely to be worth it if the price is right.

A Microsoft representative gave Joystiq the likely date release to be May the 31st.

Age of Empires' Twitter account, check here for updates on content to expect and pricing info

View Joystiq's preview of the game here