Years ago, the Voltron Force ultimately defeated Prince Lotor, resulting in h...
Voltron: The Third Dimension
Season 1 Episode 3: Building the Forces of Doom
Lotor holds a phony peace summit with Zarkon and Keith, with Lance secretly coming along. Lotor uses a crystal Haggar gave him to enter the Dream Dimension. Lance pursues and finds Haggar has been looking after Castle Doom all this time.