I am looking for A exteria of an evil castle or structure that a demon king would live in if he were to take over the world.  anyone know of any out there?

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Backgrounds:

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3dCommune has many fine castles, and castle backgrounds, in
freestuff.  Just go to

http://www.3dcommune.com/3d/free.mv?Poser_Free_Stuff

and enter castle in the search box.

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The Darklord Realm by Traveler at RuntimeDNA could be to your taste. Looks a little like Barad-dur of the Lord of the Rings movie, including the evil eye.

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You might want to check out Dark Anvils stuff too, not really castles but fantastically creepy and evil like towers, could easily be made into a castle though with a bit of imagination!

Injustice will be avenged.
Cofiwch Dryweryn.