In these couple of images you can see the rear of the fort with and without the compound wall. It's plain to see that the construction is mainly of foamcore. I would like to have lined the interior of the casemates with some kind of stone or brick effect paper or card, just for the look of the thing. The casemates are actually large enough each to accommodate one of the Armstrong RBL Guns that are on the platform above. Had i enough, I'd have loved to have had a tier of guns above and another below. Perhaps Field gund could be mounted above, instead.
That door to the right in the images is a paper-cutout of a door from a magasine pasted to a bit of card and glued in place behind a doorway cut in the foamcore.