Here's what you need:
We decided to go with spray pint to cover all of our boxes and cans because it's quick, but craft paint will work just as well as will cover the pieces with construction paper like we did with our old castle.
Once you've painted your pieces, start arranging them how you want. Once you've got a design you like, glue it together with hot glue [parent's, of course].
Then, you can start embellishing the castle. We added tissue paper brick, and silhouette windows. For instructions on how to make the tops of the spires refer to the old castle post here.
As a finishing touch, Audrie added bats strung on fishing line, a weather vane, and spooky trees.
With that we have our spooky castle, and I can say that I've officially started [and probably finished] decorating for the season.