Steam News announces the release of a demo for Defenders of Time, an alternate-history tower defense game. Here's the explanation of what this is:

Our demo for Defenders of Time is now available for free on Steam! Like the main game the demo version supports both PC and Mac. The best part of our demo is anyone that purchased Defenders of Time can invite you to their games and you can check out multiplayer!

The way it works is every person who paid for Defenders of Time can invite up to 3 other players at any time to join their Ranked or Custom matches. Otherwise, demo/free users can only play Single Player.