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Uses two-dimensional "sprites", 2D images created and used on a flat plane, as opposed to the three-dimensional models or environments found in 3D games.

Featuring aesthetics, gameplay, or themes reminiscent of classic video games. Often characterized by pixel art, chiptune music, or mechanics popularized in earlier gaming eras.

Science-fiction. Commonly including things like aliens, advanced technology, and settings of outer space or distant planets. 

The larger universe outside of Earth's atmosphere. Sometimes really, really far outside of it.

Resources for building and relating to User Interfaces, how the user interacts with your program