A good old fashioned tower defense game with a couple of interesting twists! I really like the mechanic of needing to prioritize which cannon to use at a given time, which is both because of distance to the battery as well as the element that's most prevalent in the oncoming wave. Really cool!
To be honest though, I think this game may be too difficult. I don't mean to sound ridiculous or anything, but you really don't get a lot of money in the first couple of waves at all. When wave 3 comes around and the fire elementals are there, if you don't have water guns, there is no way you're going to get through their health. From there, it continues to spiral, where the other elements are more expensive despite not necessarily doing more damage outside of the elemental weakness they're associated with. It's hard to earn money, and you can't prioritize which cannon fires at what, you have to hope it will aim at the right element. This feels less like a situation where a strategy is needed to overcome a challenge, and more like unfair design.
I think something that could help remedy this is if there was a way to have multiple cannons hooked up to the battery, but at a cost. For example, if each cannon attached to the battery reduced the battery's maximum "capacity" (or health, as it were) by 10 or something, not only would this mean you'd only have so much firepower to work with, you'd also need to decide how much extra damage is worth reducing the battery's health for. Obviously you don't have to take that advice or anything, just an idea.
Either way, I think this was a really interesting game with some great ideas, great visuals, and I enjoyed it overall!