Here's a template for the columns under the Helm's Deep keep facade:
...and here's my work in progress in building the master parts for the mold from evergreen styrene bits:
I also scribed some floor tiling detail to the main gate level floor:
I need to fit the tower in, so I started also work on that by cutting a wedge-shaped slice from a suitable cardboard tube and force-tapering the tube so that it's narrower at the top end.
I glued some foam inside near the bottom - I need to cut a section off to make the tower fit snugly against the Hornburg supports, and I fear that even with the foam glued inside, the tube will want to stretch back to its normal un-tapered form... we'll see.
@OMW, thanks for the great screen shot of the Great Hall, I've been meaning to get one but you spared me the trouble. I understand your hesitation with the curved Deeping Wall done with Hirst blocks, but if you do it long enough, the curvature should be mild enough for you to be able to build the wall from straight, regular blocks. At one point I also toyed with the idea of curving the wall builder mold just slightly when the plaster has hardened a bit - that should produce curved wall sections... but I've never tried that in practice.
My plans call for only one tower along the Deeping Wall, very near the stairs to Hornburg (I will roughly try to enlarge the Weta model of the movie Helm's Deep). Tolkien's own drawing seems to show four small towers - where did you see / read about the second tower at the far end? I could not find a reference to it in the novel.
I will have a room or two plus some corridor under the front part of the Great Hall, and a bit of Glittering Caves under the back end of the hall. It makes sense in my model, so I'm not overtly concerned about absolute reality. I doubt there are detailed reference models of Helm's Deep in existence with e.g. all the toilets mapped in
The battering ram in your pic is headed for the Great Hall door. I assume the stables have a door as well; I have not seen it in any gameplay video or in the film, but its existence makes sense and I will certainly build one for my model. The door & stables will be on the main gate floor, reachable if you take a right after entering via the main gate and passing the short passage between the gate towers. I will post regularly, so hopefully the plans will become clear once the model takes shape...