Castle Grayskull is a fortress located on the fictional planet Eternia. It forms a central location in the Masters of the Universe toy/comic/animation universe and also appears in the 1987 live action adaptation. The concept is credited to Donald F. Glut. The toy set was invented by Roger Sweet. Castle Grayskull was a "major feature of Mattel's line", and was "one of the most famous playsets of all time". Gary S. Cross, professor of modern history at Pennsylvania State University, writes that Castle Grayskull is emblematic of the making of a boy-man culture beginning in the 1980s.