You got a Good Concept , it just need improvement , like the zombies got double Health you have to think twice solving a math problem and then the undead is dead , and before enter the farm you have upgarade to defend your farm or an equaction helper. simply you have a good concept you just need to improve it , add better graphic , make the game more scarier and stuff .~.
Not bad as a training. First I meh'd, but now I aktuelly like mix of difficulty in each level. Sadly spaming the keyline 1,2,3,...,0 leads to easy victory without thinking. The music fits the purpose, but would be better if looping freely and not restarting at every level.