Death's Castle is a location in the Land of the Dead. It first appears in the episode "Death in Bloom." Finn brings Jake there to reclaim the soul of the Princess Plant and Jake's memory. It is the residence of Death.
The inside of the castle is a zen garden that Death tends to. Instead of rocks in the sand there are skulls of various animals. Some of the skulls are huge while others are big. One such huge skull has a bunch of smaller skulls in it showing at the eye and a dead tree protruding from the other eye. Death has etched circles in the sand around all of the skulls. The River of Forgetfulness has a narrow path through the zen garden surrounded by what appears to be concrete. At a point it has a traditional Japanese garden style bridge crossing it. Death's castle also features a giant stage with a variety of musical instruments as well as a saw.
Death's Castle is starkly different from the rest of the Land of the Dead in that it is made purely of light. This is probably a reference to Persephone's Garden, which is the only bright portion of the Underworld according to Greek mythology.