I know interpreting other people's sketches etc. is fraught, but I think jthomlin may have overlooked the rear curtain wall (see highlighted area) which extends from where the Deeping wall reaches the summit of the Hornrock, back around the rear edge of the rock and joins the cliff face to the rear of the Hornburg. This cliff face appears to have been scooped out creating a kind of amphitheatre shape into which the Hornburg was constructed, but as a free standing structure. I know the proportions of my sketch are off...

This is how I read the sketch with the additional wall marked 'X'.

The junction between the Deeping Wall and the round curtain walls of the Hornburg is more pronounced than my original sketch (Phase 2). This suggests the Hornburg circuit was freestanding and the Deeping Wall abutted it, rather than merely being an extension of it at the top of the Hornrock.
I'm not altogether convinced creating a bespoke Helm's Deep is necessarily going to work for me. Since I'm likely to go down the modular route (for storage considerations as much as anything else), I may follow Doug Larsen's route and model a freestanding castle that is broadly representative of Tolkien's concept, but with mods that mean I can morph it into a freestanding castle for GoT.