The forest consists of two parts. The first part is full of tall and old trees, with their network of roots everwhere. The other part is the ancient castle that the royal family of Soleanna lived in until the accident during the Solaris Project. The latter is guarded by the priests who offer trials, and the ones who complete them will be allowed access to the castle through an Eagle. The forest is also shown to be in a mountainous area, as there are several caves in the enclosing mountain walls. In both ends of the Forest there are two doors that connect to either New City or Castle Town.
The Forest was originally the site of the previous castle of the royal family of Soleanna, until the accident during the Solaris Project ruined the whole castle, leaving the Forest and the ruins as just a place for sightseeing. In Sonic's story, Sonic the Hedgehog first came to the Forest after defeating the Egg Genesis where he escape into Tropical Resort with Elise. However, he later returned to reach Kingdom Valley, after completing the three trials given by the priests.
In the Last Story, both Elise and Sonic were going through the Forest. However, Mephiles used his Chaos Emerald to distract them, and from behind, he impaled Sonic and killed him. After the final battle with Solaris, it is unknown what is the state of the forest is as the incident during the Solaris Project was erased from history.
It is the last Adventure field that Sonic is able to enter, as he must complete his entire Episode before he is allowed to enter the area (except during the short time when carrying Elise in it.)
Of all the Adventure Fields in the Game, it is the largest and as such one of the emptiest in the entire game.
If one causes a glitch to get Sonic into the area early, an area could be found in the center containing the 2-Dimensional objects that are used during the Race with Sonic Man and other Highway side-missions. There is nothing else one can do here, as the doors do not show up nor do level entrances if one does this glitch.
Of all characters to enter it, Shadow is the only one to not go through the Trials of Soleanna in order to reach his version of Kingdom Valley.