I have procured foam, and have a rough idea how I want to build the bastion which I should have in my hands tomorrow.
Here you can see the 3 tiers and the Aegis defense line
the L shape represents the outline of the bastion
Here you can get a good look at the elevation, the paper is 5" to the lower level and 10" to the top.
From what I can tell, a bastion should be around 7"-8" tall
The blue will be the water level around the bastion, so the entrances will have to be raised up a bit.
Once the glue dries I'll get to shaping the foam some.