King Austin's castle is the residence of King Austin. It is shown more frequently than any other imaginary location in the series.
The castle is predominantly gray. It has three windows and a wooden front door. Several chambers of the castle have blue conical roofs and yellow flags on top of them.
King Austin's castle has appeared in five episodes of The Backyardigans. It is first visited in "Knights Are Brave and Strong". In "Who Goes There?", it is seen in an oil painting behind Austicles's Greek urn. In "What's Bugging You?", it is shown in a photograph inside Lady Tasha's house. In "To the Center of the Earth", an illustration of the castle can be seen in a cave underneath Earth's crust. A photograph of the castle also appears in "Follow the Feather".