I think if you went marble or swirl, you’d lose the detail of the castle—I’d stick with a “solid”, myself. One cake I know will work is the Perfect All American Chocolate Butter Cake (from The Cake Bible - TCB), because Rose doubles (I think—don’t have my book handy) it to make the Chocolate Velvet Cake in Rose’s Heavenly Cakes (RHC). Most butter cakes will work as bundts—it’s the other way around that can get sort of sketchy!!!
Also, in RHC (if you can get it from your library, perhaps) in the wedding cake section is a cake made in the stadium pan. It’s the Chocolate Domingo Cake (from TCB) that has diced, very fudgy brownies mixed into it. If you can’t get RHC, you can make your favorite fudgy brownies (not cake-like, but fudgy), let them get good and cold so you can dice them without them sticking everywhere, mix them into the domingo batter and then bake. If you like the concept, you might be able to go with “blondies” and a yellow cake!
Also, if you want a color contrast but also want to preserve all the castle detail, you could bake a yellow cake and then pour a chocolate laquer glaze over it. I expect it would be slow going to catch all the various “things” of the castle, but it would make it look very spooky, I’ll bet!!!!
Another thought is, if you’re good at piping (and I’m not), is you could make a solid cake of any color/flavor and then pipe details in frosting of any color/flavor. Maybe even put some colored jimmies in the windows to look like stained glass!!!
Looking forward to hearing about it, whatever you do!!