I reeeeeally like this idea and the concept. I think you can make a greate game out of this with a bit of tweaking! Right now I think it's a bit too random which makes it frustrating (or maybe I'm not getting something) - but without knowing where enemies will spawn and the random amount of defense cards it feels like its basically pure chance if I'm able to win a round or not. I also agree with earlier comments - I feel this could work really well with deck building!
Great job!