The Doctor's Castle is a repurposed medieval castle, formerly inhabited by the Mad Doctor.
This looming medieval castle built atop a skull-like cliff, which may have been built by the Mad Duke of Duckburg, was reinvested in 1933 by the mysterious Mad Doctor. By then, it had come to be haunted by the Skeletons, who, having nothing else to do, did the Doctor's bidding. The Doctor repaired the castle and added such modern elements as a doorbell, electric lighting and an elevator, on top of his numerous devices. What became of the Castle after the Doctor was transported to Wasteland is unknown; it is possible the Skeletons remained in what had become their home, or it might have returned to its abandoned state.