The concept is definitely interesting, I think I've seen games of this sort before, but there are not enough monsters to make the game interesting. The beast is OP, slimes can be useful when paired with a beast, and the speedbug just straight up sucks. Which is disappointing because I wanted to have a blitzkrieg attack style in the game. If I had maybe two or three more monsters and a bit more interesting level design, like needing to attack in a certain pattern or being able to collect buffs along the way or break towers for extra money that'd be all this game needs to be very fun. Your music and sprites are spot on though.