the reason all the giant bombs are in the core is because once the hogs are finished funneling around the second layer, they will hop over the wall and get blasted by the bomb. by the time they get to the core there should be only about 60% of the original hogs left. I had the wizard towers moved as well. i placed them towards the top like this
http://i.imgur.com/7hUi0Rk.png?1
The reason i moved them up is because now they will be better protected by the canons and will not be the first defense to go down if the wall is hopped by hogs. The thing i like most about the base is that i can change the placement of the towers freely and still have good coverage around the base. Thanks for the feedback!