A demo of the game is now available as a free version of the game. The only differences with the release are:
There is a limitation of rounds per day in the solo play.
It encourages to buy the game with a button in the main menu.
The original plan for multiplayer was to allow players with the free demo version and the release version to play together, when multiplayer will be available. But it seems it won't be possible because how Steam lobbies work.