This demo is actually a full game, exactly how it looked on 16th October 2015, when Thea was still in Early Access.
The game has been greatly improved from that time, so this demo does not represent the quality of the full product. But it should help you decide if the gameplay principles suit your taste.

If you purchase the game you will have free access to all Thea's DLCs ever released, which add co-op play, localization to DE, RU, PL, and FR, new Giants storyline, bonus events and events editor and well over 150 updates that improve every aspect of the game.