Bricks = sugar cubes Mortar = royal icing (1 lb. confectioner's sugar + 1 tsp. cream of tartar + 1/2 c. water blended til smoooooooth, no lumps)
Build up your walls, painting the icing in between each brick/sugar cube. When the royal icing hardens, the bricks will be stuck to each other pretty firmly, but still don't move them or jiggle the buildings too much. It's delicate.