I've been doing some basic research into Iron Age hillforts to try to get an idea of what a basic, early-style enclosure would look like. (Also I thought that was a really pretentious way of saying, 'so, there's these videos on YouTube....')
One thing that strikes me about the designs is that they tend to be very....open. Lots of space for outdoor activities, grazing animals, etc. Sure, there are buildings, and there's an elaborately planned gateway structure. But...it's not 'castle = keep + walls' at all.
Now that stuff is all really old, and wood rots, so there's a lot of guesswork that goes into what was actually built. The earth-moving is the most enduring, and there's pottery fragments. But if these are going to be the first defensive walls to go up....Formenos should be somewhat reminiscent of primitive hillforts, even if the craftsmanship is really advanced. Maybe?
Some examples:
Welsh:
English:
Swedish (though rebuilt later, so not really an iron age example):
I am happy to have the Noldor work in stone, as that seems to be their favorite, and they have experience now. We don't have to make them primitive, not while building Formenos. I don't dislike Masada - I think it's a great starting point! But....maybe give them some more land and outdoor space to work with? Of course, if we want them to seem cramped/trapped in there....it's a balance. But the courtyard should be large enough to practice troop maneuvers for Fëanor's entire force.
And I know we can't have anything as elaborate as this: