You can do a 16" gun on a KE if you've got good enough weight techs (and 16" guns unlocked, of course!). It's utterly useless, but still a very funny design--especially if you give it a name like "Elefant!'

I know that fortifications make ground combat much harder for the invading force, so it would stand to reason that fortifications would help you crush rebels, as well.

All I know is that I've never lost a colony that has fortifications in it--so maybe next time you get the rebellion notification ...