Nice idea for a game. Fits the Game Jam theme very well.
I do like the build up income stream then launch a final mega attack to win style. Actually very similar to my own submission in that regard.
For improvements, there's the adding art/ui/audio assets (as you know). I'd also suggest either adding the option to bulk buy soldiers or hold down mouse to buy them. My mouse took quite the beating playing this game.
Good submission. Well done for making a game given your personal time constraints!