This article is from the Dissociation FAQ, by Discord (email@example.com) with numerous contributions by others.
An internal landscape refers to how the multiple "sees" the inside
of hir mind. It's where the alters live. It can vary greatly
both from multiple to multiple as well as from alter to alter
(for instance, one alter might live in a castle with a moat
and a drawn draw-bridge, whereas one "right next door" might live
in a city block). These are their internal homes, where they go when
they are not helping run things, or when they are hiding, or whatever.
Some people think that their internal landcape is actually a link to the
Astral Plane. Some people don't believe there is any such thing. I'm not
sure it matters.
Not all multiples have an internal landscape. (For that matter, not
everyone with an internal landscape is multiple.)