For Red Alert Zero (currently on hold);
The Grand Cannon is basically a massive defensive howitzer; firing high-powered shells over VERY long distances at faraway targets;
My first version was the one of the left (basically a massive turret).
The second version on the right (a fixed gun with a 90 degree field of vision).
I'm going to do a third based on the second with some corrections; basically a forward-facing gun (handy as it means the gun won't pivot backwards and blow up a target that snuck into my base- having said that, there could be an individual upgrade to convert it into a 360 degree turret for players that want to start placing them in ambush positions!), and flanked by shielded crew stations (namely, a gunner sitting down on one side, and on the other side, a commander standing up with his binoculars).
But that's coming soon- right now I'm just finishing up a large backlog!