I mean this is just great game isn't it! Amazing that you made it in 1 Month. Definitely going to be one of the top submissions.
To try and give some constructive feedback (I'm nitpicking here), it would be nice if instead of buying the item as soon as you drag it out of its shop slot, you buy it when you place it instead. Couple times I picked up an item to see how it fits (I know it shows its size in the popup, but I auto try fit it) and accidentally bought it.
Another thing I'd note is the very first level feels a little bit slow. Maybe up the basic weapon base damage by 1?
As I said, these are very minor points. Suberb game. Full marks all round. Congratulations!