Range sucks. For how weak you make the king and the upgrades, the speed the zombies get greater in number and power is nearly impossible to keep up with.
Why not make a Thing-Thing arena like this? Thing-Thing Arena Defense? Have a money system that you get when you kill Things (many different things to kill), then be able to buy fancy weapons and defenses. The point being to protect yourself.
there should be a version that instead of pre-made compounds you use the periodic table and do more chemistry-like things, with some important compounds like Water (H2O) and Saltpeter (KNO3).