Frosty Castle is a frozen palace in the Mushroom Kingdom. The only route to Frosty Castle is entering from Chilly Waters or by using a warp pipe. Wario and Waluigi traveled to this palace during the events of Mario & Luigi: Sibling Rivalry. Frosty Castle is inhabited by King Frosty himself and his loyal servants. During their trials in Frosty Castle, Mario and Luigi were eventually brought here by a pipe that the Wario bros. had opened.
The palace is made almost entirely of snow and ice (due to the cold reaction bad jokes get), making it resemble the Crystal Palace from Paper Mario.
The Warp Pipe in the game is located at the entrance in Frosty Castle.