Infinite money.
Box Dude TD 5 is a space-y tower defence game where you have to fight off the invasion of weird square alien...things. The game features 4 maps, 8 turrets, 10 medals, and 4 online leader boards.
Click a turret on the top bar to buy it. Then, simply click anywhere within the white boundary to place it. When you're ready click the 'GO' button to start the round. Every time you defeat an enemy, you'll gain more credits, which you can spend on different turrets to better your defences.