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The Emerald City is the seventh level in Keen 1. It is a big level, built out mostly by green (or emerald) bricks. It is full of hazards (fire, and mostly plants: spike, strangling and Pat Pats). Its green-ness may come from the abundance of plants. It may also be some sort of greenhouse, or the equivalent of a Martian park.
There are also some strange Martian devices on the background (crystal cases and antennas), and it features the first yellow tunnel found on the game (the Red Maze City is full of them). It features all the known Martians and their robots.
See also List of Keen 1 Levels.