The Great Forest is a forest which is located outside the city of Trolberg, close to where Hilda's old house used to be. The forest was first mentioned by name in "Chapter 11: The House in the Woods", when Hilda got lost here following her adventure with the Weather Spirits.
The Great Forest is made up of ironpines, a species of tree which resemble ordinary pine trees yet grow to the height of redwoods. It has many features common to forests, such as grass, butterflies, rocks, and moss.
Forest Giants are residents of the forest, though they don't seem to have any sort of society. Wood Man lives in his house near the edge of the area. The Great Forest is also part of The Northern Counties as previously stated, and thus has a population of Elves.
The House in the woods is located in the Great Forest.