The Lost City
The Lost City is a long level, being even longer than Native Fortress. The level contains an entirely new batch of enemies, including small crawling lizards, and large, jumping lizards. Both need to be jumped on to be killed. There are also walls that slide in and out, which can easily push you off of the edge. This level also has the very first colored gem.
Since the level is so long, it is tough to get the gem. It doesn't require anything, but staying alive, while breaking all of the tough boxes definitely will prove a challenge. Fortunately, once you pull it off, you will acquire the Green Gem, enabling you to finish Jungle Rollers and unlock a secret path in Castle Machinery, a much later level.