Skeleton Piles (not to be confused with Graveyards) are special areas that appear in The Curse of Castle Blackburn stages of the Steam, iOS, and Android versions of Kingdom Rush. These areas spawn Skeletons and Skeleton Knights sporadically.
The area is just some scattered bones. Skeletons and Skeleton Knights spawn from the terrain of these areas and go towards the exit. Their spawning instances occur at fixed times after the start of certain waves.