To answer your questions Dobbs.
Firstly I bought this foam for making my Fort Ticondoroga from a firm which insulated the insides od caravans. For large 25 mm scale structures it was fine but too thick and unwieldy for this finer work so I went to a model shop who did the same foam in smaller sizes.
They call it modellers foam but it is exactly the same as the insulation foam.

From other pictures I have of the fort it seems as if it was a composite of two sides of a standard star fort with a third wall following the contours of the valley with the houses in it. That wall was slightly angled with a double gatehouse at the central point and what seem to be small round towers at the extreme ends. Of any rear bastions there seems to be no evidence.
I am going to do the best I can with what is available. After all it is for gaming and I have already compromised on rthe island overall size which should scale to about 400 m wide to 500 m long. At 1200 scale that is just too big for my games table.
Rob.