Witch's Castle is the nickname for a ruined stone comfort station near the meeting of the Lower Macleay and Wildwood trails in Forest Park. The building was damaged in the 1962 Columbus Day Storm and has been left as a mossy, half-broken landmark in the woods.
It is not a castle, and that is part of the charm. Treat it as a short detour on a Forest Park walk rather than a standalone sight, with Pittock Mansion or the Macleay trail making the visit feel more complete.