This game is quite addictive with its upgrade systems,level settings to adjust difficulty(although it is not a new idea but a good addition anyway)and the fact that completed levels could be replayed to earn more points for upgrades before moving on.
(1)However,it does not fully engage the player in using much strategy besides deciding and placing defenses in the best places to cause the most damage on attackers.Maybe a Fairyland campaign where players get to go on the offensive and sometimes defensive against the kidnappers could be included with more types of units.This will not only make the game more interesting,but also makes the game suit the title better(this way,the demons are fighting with Fairyland with both sides and defending.)It may also suit picky players who always dread 'another tower defense game' as it is different from the other examples of defense games stated in the comment though it will take more time and effort.
(2)There could be more spells(to freeze enemies for a short time or to zombify attackers)and defenses and traps like pits and mines to add more variety to the game.
Overall,this game is quite addictive even as something inspired by other defense games.Keep it up!