If the castle model is big and expansive enough you could just leave it where it is, have an opening to walk through, and then you could walk right inside, checking collision against walls and all that. But if you want there to be a nice big whatitlookslikeoutside model and then have something much more expansive inside, then use the one you have already, then make a bunch of twisty passages in a modeller for the inside. Then you when you detect that you've walked into the castle you can get rid of the outside castle object and put in all the hallways and walk around in there.
Crazy Donut Productions, Current Project: KillZone
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