I guess I should give credit to peskyninja, cuz the second post says "you can use 1 tile bridges". Initially I was going to use bridges, but I decided on walls instead.
I read that using diagonal walls, example
x=wall
x x x
x x
x x x
x x
has some benefits as well.
I also had another idea.
In all my forts, I have a walled off area that my dwarves have claimed. I then setup catwalk bridges from outside of the walled off area that lead into the fort with a spiked moat beneath. During the event of a siege, I use pulled levers to "repeat" the bridges on/off.
I have no (melee) dwarves posted to keep a view on incoming invaders for fear they might run into the repeating bridges.
Instead, at the end of the catwalk bridges, I have a stairway leading (1 z level) down into the barracks. This means my melee dwarves don't see the enemy until there are in the barracks, and also means the enemy has to fight through the barracks before entering further into the fort.
I could further improve upon this by setting up diagonal walls, or doors a z level above the barracks, somewhere between the stairwell, and the bridges, which would further prevent viewing of any enemies on the bridges. This way if I have to, I can post dwarves on the same z level as the bridges.
I do have "lookouts". Mainly marksdwarves that are a z level above the catwalk bridges, with access stairwells that go directly to the barracks (just in case they try to chase after the enemy, they have a longer route to go through, which gives me more time to react).