Fortress Valley is a valley about the same size as the Great Valley. It is located a few miles away. Unlike the Great Valley however, there is only one sure-fire entrance (which doesn't apply to Flyers), and that is a single tunnel way littered in boobie traps and protected by cacti plants surrounding it.
On the inside, the valley appears to be peaceful, much like the Great Valley. There are herbivores who live here as well. The characteristics that physically seperate the Great Valley from Fortress Valley is the mountains surrounding Fortress Valley are jagged and sharp, shaped like a sharptooth's teeth, and the waterfall that blesses the valley with water comes from an opening in a rock formation that has an uncanny resemblance to a sharptooth's head.
But despite it's beauty, Fortress Valley is no paradise, at least for plant-eaters. Plant-eaters have no where to run from the sharpteeth, and are kept as slaves. They are imprisoned in a cave near the center, often called the Feeding Grounds or Feeding Area. They are guarded by two of the largest sharpteeth in the area. Any of them who dare leave is immediately killed by any sharptooth who sees them.
Fortress Valley is a sharptooth paradise, but it wasn't always this dark and horrible. Fortress Valley was once a free land for both sharptooth and leaf-eater. But that all changed when Slasher came and took over the place. Under his rule, leaf-eaters were stripped of all rights and were forced to do what the sharptooth wanted them to. This means that if they were called to the Feeding Grounds to be eaten, then they must go, or suffer a punishment worse than death.
Fortress Valley is where Chomper grew up with Slasher, and learned the ways of the killer and murderous sharpteeth.