Kingdom Valley is an Action Stage in Sonic the Hedgehog (2006). It is north-east of Soleanna and once housed its old kings. In the game, Kingdom Valley is the site of one of Doctor Eggman's bases. Kingdom Valley abounds with water so the ancient ruins and weak wood platforms are not the best footholds.
In Sonic's story, he and Silver are in pursuit of the Egg Carrier before it will explode, which will kill both Eggman and Princess Elise. The first part is Sonic running through ruins and trying to reach the Egg Carrier. The second part is Silver helping Sonic to reach Elise. It utilizes a High-Speed area at the end which takes place in a set of ruins encircled by rushing water that Sonic also needs to run along. This is the fourth and final high-speed area, after Wave Ocean, Crisis City and Radical Train.
In Shadow's story, he and Rouge go to Kingdom Valley because it is the G.U.N rendezvous point for the Scepter of Darkness, which Rouge obtained in White Acropolis. Shadow must use a glider to pass through some of the level and a Hovercraft to cover a large water body. Shadow is ambushed by Dr. Eggman at the extraction point, which leads to Mephiles being released and taking Shadow's form. Mephiles then sent Shadow and Rouge into the future to Crisis City. Shadow later returns here to find the red Emerald in the End of the World.
In Silver's story, he, Sonic, and Blaze chase after the Egg Carrier to rescue Princess Elise from the soon to be destroyed Carrier. Sonic then travels to the past again to successfully save Elise and gives Silver his Chaos Emerald to return to the future.
Silver's act is considered the hardest. However, since Sonic's high-speed area requires careful jumping and timing, Sonic's act can be considered the most difficult.
In Sonic's story, the player can skip over Silver's part of Kingdom Valley and finish the level without him by using the Purple Gem.