A few thoughts:
-I haven't been to any big fests, so I pretty much build for the camera. As such, my creations are a lot like movie studios. For exterior scenes my buildings are empty shells (or even just facades), for interiors I do three walls and no ceilings.
-If you're going to build a large castle for exterior only, another good option on the inside is DUPLO bricks. You can sometimes get great deals at garage sales etc since people usually only want them for a couple of years. My neighbor's kids are a few years older than mine, and he was going to give a bunch of stuff to GoodWill, and asked me if I wanted any of it for my kids. We doubled our DUPLO collection that day.
To interface from DUPLO to System, just put a System 2x4 brick over top of two studs of DUPLO. (Or, shhh, you could use clone brand bricks on the interior.)
-One suggestion is for your large castle to have cut-away walls at a few points so you can see in to a few rooms. That way you can do a little bit of interior work, and have it on display for visitors to peek in, but won't be obliged to fully furnish the whole thing.