While the trailer may give a fair example of game play elements and the game may be worth the asking price I don't see a playable demo to check if it works with a given system configuration.
Given its looks like a wave based 2d action game I would think having a heavily cut down version of the game with just the basic starting equipment and the first wave on the first map would make for an adequate demo to check if the game works - given all the materials and coding is done for the full game finding the section to duplicate for use as a demo would be the hardest part I would think (based on how the developer describes the games code).