In TOS, they often zoom in on the captain sitting in his chair on the bridge
from a viewport. This indicates that the viewport is actually a window that
you can physically see out of... potentially the bridge might actually be able
to "rotate" which would give you 360 degree view of the space around you.
Although they do have "magnification" enhancements, so its possibly some sort
of Window+Screen hybrid... where the window is made of some extremely strong
material, as invented in Star Trek IV by Scotty.
All of this makes sense, as if you were to embed the bridge within
the ship, you would have to rely solely on a computer screen, which relies
on: a) sensors, and b) cameras - which can be disrupted by technology.
Something not proposed in Star Trek, but would make a lot of sense, is to
make the bridge a "mobile" module, that moves up and down through the
interior of the saucer section... kind of like a periscope. So that if the
sensors or cameras are in question, you could move the bridge module up
to take a look around.
Also... I believe the whole star ship is designed to be modularized, as you
have seen that, they are able to jettison entire modules. So each module
might even have its own force field, (i.e. sub-shield generator)
that way... in the event that the exterior shield ever went down, if you
hit a single module, it would help to mitigate the damage of each individual
module. This could explain why, when a room is damaged in a ship, you will
always see a force-field covering the exposed area.
You can see that they can create force fields in sections, all around the
ship... it makes sense then, that every deck/section has its own sub-shield
generator, which ultimately helps protect the structural integrity of the
ship.
So the best explanation is, that the bridge is not as vulnerable as you
might think. It might be positioned in a seemingly vulnerable location, but
when you factor in,
- Materials used.
- Layout of materials (i.e. bulkheads).
- Localized force fields
- Shield (global force field).
Then its clear that it is not as vulnerable. Also considering, there is
auxiliary control, which would be used as a secondary bridge, and is in a
more safer remote location.
Then consider, if the saucer section were to detach, and to make a crash
landing... (which Kirk has done at least twice). Would you rather be in the
crow's nest, or would you rather be in the heart of the ship, that will take
the brunt of the structural stress?
I'd like to think that the bridge is capable of moving up and down
levels, like a turbolift, like a periscope. That would be best.