The tower defense genre is very good at telling simple stories. The narrative serves as a good setup and the player's investment into the gameplay (not losing their progress) compels them to fight for that story.
I'd caution you all to make sure that there's a story to tell. What do you want the player to walk away with other than "pollution is bad." I'd suggest finding powerful ways to make the player empathize. A great challenge and a losing state can go a long way. In Kingdom Two Crowns, another tower defense game, you grow to hate the enemy demons after you lose to them once. Find a way to make the game actionable.
I hope there are more specifics on the gameplay at the end of this prototype. It's concerning that there is no mention of your units, how to place them, and what they do.
Here are a couple of notes specific to your plans:
- Backgrounds can take a long time for artists not specialized in making them. Having different backgrounds per phase may be a lot to ask for. If your style is pixel art, consider using a program like PixaTool to speed up the process
- The "spirit of radiation" enemy concerns me. Nuclear energy is one of our cleanest energy sources. All depictions you see of nuclear energy like barrels of green goop are propaganda. If this enemy refers to something other than nuclear energy, you must make that abundantly clear. I think you should contact an environmentalist for a better idea of the problems which are causing climate change.