Worlds

A world is a complete stage with a particular theme. The worlds can be played back-to-back in Arcade mode; once the player has reached a world in Arcade, it can be played separately in Single World mode.

A world consists of a minimum of fifteen levels, typically:
 * thirteen regular levels with a varying number of humans, enemies, and secrets
 * a fourteenth level that acts as a bridge to the fifteenth level, with only a couple of humans and Harvesters
 * a fifteenth level that culminates in a boss fight

Each world also contains a number of secret levels, accessible by breaking arcade cabinets hidden in specific levels. Accessing the secret exits is not required to complete the world, but is necessary in order to collect all humans and secrets.

With the exception of Space Station, which is only playable on Experienced difficulty or higher, all worlds are available on any difficulty.