First, to make the towers, you'll need a bunch of small rounds. I used a bunch of 6" rounds because I wasn't sure what size I would need, but I could have used 5" ones. Crumb coat the main square cake, but you can skip the corners. I used a 10" square.
Cut off the corners...
Cut the rounds in half and put them up against the corners. Use icing to glue them onto the corners. They should be the same height as the main part of the cake.
Trim the rounds to the size that you want the towers to be, and so they line up with the cut are on the main cake.
Add another piece on top of the half-round so that the tower will be taller than the main part of the cake. Ice the towers.
Cover the cake with fondant or modelling chocolate (which is what I used for this cake.) Make sure that the towers are attached to the main part of the cake with the panels of the fondant to keep them in place. Wrapping the tower with one long panel of fondant that then extends onto the sides of the cake is the best way to go.
Decorate however you want!
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA