Today I've implemented the chain-reaction part of the game. In the battles, if you defeat one of the aliens, then it will generate shockwaves that damage the aliens that were standing on the left and right. Even better, if a shockwave manages to defeat an alien it will create a stronger shockwave. And, you guessed it, this can happen one more time, creating the strongest shockwave. I intent this to be the only way to defeat certain enemies later in the game.
I've also added an experience system to the game. So if you defeat enough aliens Harris will level-up and gain extra hitpoints and attack power. Although I'm not sure increasing the attack power this way is a good idea. It seems like you become overpowered really quickly this way.
There are also objects in the game now that can be interacted with. Simply walk up to them and press V. Lastly the game-over screen now displays "GAME OVER" (surprising, I know) and asks if you want to continue.
Tomorrow I plan to implement the boss for this game and change the level to roughly how it should look like in the end. That way I'll have something to submit already.
Controls: Arrow keys for movement, V for confirm, C for cancel.