The cannon itself was a pretty easy build, but then I chose to add a loading crane. The crane is made up of bits including 2 Ork chain weapons and some angle iron from Plastruct. I am not finished with this part yet, wanting to add a work platform with controls and of course some sort of hook.
This is all going to go on a large tower at some point so I tried to make sure I could resize it once I have decided what my defensive towers are going to look like.
As I said the cannon itself is an easy build, but it is also huge compared to an Earth Shaker from Games Workshop the piston underneath is hard to see but it works, I even have a hole in the back where air could be tubed in somehow.