This is really promising, it unfortunately gets too easy and I was able to leave a defense unattended and it finished the game. As a next step you would need to look at ramping up the bug strength and then having upgrades to the defenses too so that people can improve those to match...
...also consider the shape/area of attack of the defenses. e.g. at the moment both the spray and the glue are area-of-effect, which means that they work just as well whether 1 or 20 bugs are coming. More of a tradeoff between damage and rate of fire and number of targets would give the player more strategic choice.
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