Now that you have a moving player, the last two things you'll need to add before you have something that can be considered a "game" are enemies and bullets. This article will cover creating an array of enemies and introduce an important concept needed to handle bullets: a sprite group.
Before getting started, be sure to download the part 3 example files. This is a different file than those used in the previous gaming articles, so you'll need to download the file using the above link. Note that all pixel art (such as the player, aliens and bullets) were created by Pixel_Outlaw.