Hard mode only clicked when I chained incredible powers between tower cooldowns; the final boss rush hits as soon as you lose focus.

Rotating through deadly weapons while towers recharge makes surviving the crash feel tactical instead of frantic.

Normal mode is my warm-up, but jumping into the third difficulty really tests how long you can hold out before the mother ship arrives.

Dirty Earthlings being free on FlashGamesBox means I can run a few levels any time I want a tower-defense fix.

Staggering powers across the mid levels keeps the soldiers from surrounding you, and saving the biggest blast for the boss is essential.