I am making a castle cake and want to use a dummy layer for the base b/c it doesn't need to serve that many, but I want two layers.

Can you use styrofoam sheets that you see at craft stores, cut them to shape and hot glue two together to make the dummy layer the right height?

I think you may find that the styrofoam at Walmart, etc will cost you more that buying a true cake dummy.

If you use regular styrofoam, I would suggest covering it with food safe plastic like Glad Press'n'Seal and then spread piping gel on it and then cover it with fondant.

HTH's

Here a site that are reasonable in prices and shipping.

http://www.dallas-foam.com/store/cakedummies.html

All different sizes and thickness.