Cool idea!
The tower defence genres fits the theme perfectly, however I think the implementation of upgrading the towers might have helped push the theme a bit further.
Also I didn't realise what the buttons / numbers meant, some simple icons (Even like a heart next to the health) can go a long way for game jam entries! Things like that help the player a lot I tend to find (not that I often keep to it myself in my previous games).
With all that said it's a well made entry and I definitely like the idea...
Let me know (message me on itch or something) if you update it to have a devlog so I can rate you a bit higher on that end!