I like the concept & visuals a lot! I think the gameplay has a lot of potential but I did have a few issues:
The text boxes were generally very small, and sometimes they wouldn't disappear and would be in the way during the waves.
Maybe I missed it but I wish there was a way to see fuel costs after the explanation, I forgot the cost for a few of them.
The difficulty ramps REALLY quickly once you unlock the towers, before you really have a chance to try them out.
Other than those things I really liked it, great job!