I am going to carve a cake in the shape of Wrigley Field Baseball Field. I know that I will start with a bottom cake and then just add the sides for the sides of the stadium. My question though is how do you keep cake from falling apart when you start carving into the correct shape? Do you freeze the cake first?

Yes, a frozen cake is waaay easier to carve! Oh, and use a long serrated knife.

I always freeze my cakes. It makes it way more stable for carving. Start cutting as soon as you bring it out of the freezer. Between that and buttercream for "glue", you should have no problem. And don't forget to dowel if it needs it! Good luck, and please post pics!