Control Joe by using your W/S keys to move up and down along the gate that you are defending. Use your A/D keys or your mouse scroll-wheel to cycle through your available weapons. Use your weapon by clicking on your target. The level (or wave) ends when you've killed all of the enemies. The game ends when all waves are completed or if New Hammerston's gate health drops to zero.

Your friends, Bernie and Jess, attack automatically. Harry the robot's abilities are activated by clicking on the buttons located on the lower-left of the game screen. Harry must have enough energy to perform an ability.

At the end of each wave, you earn credits and you re-enter the colony. Here, you can upgrade your equipment and colony-related items. Spend your credits as you see fit!

Some advanced upgrades require a certain number of citizens to purchase. You gain citizens by saving survivors during a wave. Survivors take damage as they run towards the gate, but you can regenerate their health by clicking on them, until the reach the gate. Once they've reached the gate, they are citizens. More citizens also means higher passive gate health regeneration. 

When the game ends, you have the option to submit your high scores and see how you stack up against players around the world!