Here it is:
I used my tried and true marshmallow fondant recipe from here, and made twice as much fondant as I needed.
I made gingerbread cookies for the buildings, which worked well, except I did not cool them appropriately, and they were slightly bent! Not good for the overall appearance. Points off!
And I used a royal frosting recipe from The King Arthur Flour Cookie Companion.
The buttercream icing that I used between layers and attempted to use as glue (note the qualifier here, attempted,) was the Wilton recipe. The webs are chocolate. Very easy and good results!
So I was pleased with the overall idea of the cake, and the individual parts of the cake, but the cake, when put together, was a bit of a failure. Here's why:
1. I did not realize how heavy 4 layers of cake would be. They smooshed they bottom layer, thereby causing a leaning tower of Pisa look. We had one crash during the construction of the cake tower. I ended up using wooden dowels to keep it steady, but was too lazy to try to add cake to wedge the tower back to straightness.
2. Gingerbread cookies need to cool completely or they will warp. My buildings were all a little bent.
3. I have no idea how to properly attach cookies to cake. Maybe I should have removed the fondant? Or maybe the buttercream was not a good adhesive. Twice with the cookie buildings falling off. Once they crashed into the rest of the gingerbread cookies. Royal icing transfer emergency!
4. Not sure I've got the whole royal icing thing down. Mine stayed sticky for days after it was applied to the cookies! It was so sticky, I tried using it to glue the buildings but no success there, either!
The birthday boy didn't notice these shortcomings--the crashes were very exciting and funny, and I guess all's well that ends well!