I decided to just push ahead and start cutting foam, instead of the complex geometry stuff. It seems to have worked out so far, what the heck? So I cut three layers of foam to the outline of the bastion and then cut it again at the draft angle of the walls I had finished earlier. A couple of mistakes and a whole mess of hot glue and I have a bastion. All in less time than the math would have taken.
those are 28mm WSS figures from Wargames Factory
this is going to be a HUGE model when it is finished
the Mantanzas Fort looks tiny sitting inside this thing
and in relative terms the Castillo de San Marcos was small,
Spanish forts in Puerto Rico and Cuba are MUCH larger