On Cake Wars they often use rice crispy treats for elements of novelty cakes that they then cover in fondant. Sorry I don't have recipe but remember my grandma making it by melting toffee, butter and marshmallows before adding rice crispies. This might work though I've never tried it. The bumpy texture may look really good as stone too. HTH
Its DD's birthday in a fortnight and I'm still deciding what cake to do for her. She loves Ben & Holly so I thought perhaps I could do the castle. To do the castle, I thought I'd make a traybake, use a cutter to cut circles and then layer it up using buttercream & cut it horizontally rather than vertically. How could I do the grey walls which go around the garden area? Any ideas? For the garden itself, I thought I could either do green fondant icing or dye some dessicated coconut.