I really like the idea for this game, it took a couple of minutes to get my head around how everything worked, but after it clicked I had a lot of fun. A few small improvements/ criticisms:
- The overlay text on the transparent white background can be hard to read (I think the background should be darker)
- It would be nice to see the money/defence including upgrades on the planets (not just the base values of the planet)
- The last upgrades that you can get didn't seem much more powerful than the others/ weren't that necessary as I managed to beat the game without using many of them (and all of my planets were fully upgraded after the second wave)
But overall I think this is a great game :)