I think this is a fairly good tower defense prototype. My biggest complaint would be the lack of narrative flavor. That said, I'm not really sure what half the towers are actually doing. More pressingly, the base kept lose hp for no discernible reason during my run. I still got to 12 storms, but I couldn't figure out what was damaging the base. The creatures never got close.
Other than that, I think the choice of red on blue for the text to upgrade towers was a poor choice. I get that the red is indicate loss of gold while the green is to indicate an increase, but really it just made the cost hard to read.
The options menu is a curious addition. The key config is completely indecipherable, while the listing of a "windowed" mode is something I wouldn't expect from a game I can't download.