The Ghost Zone is an alternate form of reality, home to all the ghosts, and made entirely of ectoplasm. The Ghost Zone as a dimension in time-space fluidic phase as portals can naturally form--portals between the Earth and the Ghost Zone, portals to other dimensions, and portals through time and space. The Ghost Zone features a wide variety of locations, all of them accessible through unusual doors that's shape, size, and style usually have something to do with what's through them.