Very common for your towers to simply ignore targeting the closest target, letting them just beat up your towers, for no reason. This happens without having bought any of the upgrades, that make your towers only able to target certain types.
Also, why is the magical AoE just like so obscenely more powerful than anything else? Chain lightning hits a few target for 8 ONCE at max level. Magic AoE deals 12 damage PER TICK, while usually also hitting more enemies, once you hit level 10+.