I am attempting a castle cake with a dragon on it. I also want to make a tower to stick and stand up on the castle. The general idea to create the tower is to stick Oreo biscuits together and then to fondant the outside to make it look like a stone wall tower.   Can someone please help me figure out how to stick the biscuits together? I tried to Google but couldn't find much. 
You are going to have to experiment, if you want to use oreos.  The problem is that you need somethign strong enough to allow you to roll the glued together tower in fondant.  I would try, in order:   Tempered chocolate, if you can do that; its pretty strong when fully cooled Royal icing Very thick ganache Peanut butter  