I made a mistake last night and I was hoping to fix it as I don't have time to redo it all. I was cutting out side of a castle to attach to the side of my cake. The will stick up past the cake. I cut them out of a mix of fondant and gumpaste but they didn't harden up enough to stand up on their own. Live and learn. Next time I will just use gum paste. Anyways is their a way to attach heavy cardstock to the back side to help it stand up?

You could try using royal icing to glue it to the cardstock.

Piping gel might also work

I had the same issue once and I ended up covering the back side of the 'wall' with melted chocolate (rather than cardstock - not edible); it dries fast and was sturdy enough. Just a thought....

I hadn't thought of the chocolate. I wouldn't have to worry about the bleed through either. The kids would love to eat the castle walls. I could coat the back and put them on my form to bend them. Great idea. How much chocolate did it take to stiffen it? Thanks

Just about an 1/8" layer smoothed with a knife was enough to provide enough stability for the roof and the forklift attachment (see the forklift cake in my photos) free-standing. I let it dry overnight; but i'm sure it won't take that long to dry. And you're right, the kids will eat it up