I may have whipped up a couple of concepts. Here is a very rough sketch of a hexagonal cogfot plan, with a bit of foot detail, and a 3d model of one of the constituent cogtowers.
I need to add more detail to the model. but will probably work on the skeleton of the rest of the castle first. clearly it should ultimately be as gothic as possible, but there's a limit to how much of that I'll be able to implement in sketchup.
I'm imagining a set of gun ports all around the tower. Gear teeth at the top and bottom of the bridge level would allow the bridge to remain stationary, while the tower rotates within an armoured sheath, bringing different gun ports to bear on the enemy. It can rotate at the rate it takes for the gun crews to reload their canons, so that there is always a loaded gun pointed outwards.
Additional bridges will lead inwards to the central boiler dome, which will be suspended above a mass of gears and clockwork, with pistons and cam shafts to drive the legs, and the rotation of the tower. There should maybe be six legs to mirror the hexagonal profile of the castle.
The bottom of the structure will be an armored bowl like structure, to stop anyone from getting under the fort, and attacking the clockwork from below (I'm looking at you goblins and skaven...) It will probably need some defense against aerial assault as well, maybe some angled metal plates suspended above the mechanism.
Sorry that the plan is so rough. I wasn't intending to draw up proper blueprints, but rather get the scale of the different componants straight in my head before starting on the 3d model. Once that is done I should be able to output whatever views I want, to turn into proper paintings, blueprints etc. Or, you know, get it 3d printed at 28mm scale...