Not sure what would cause your marksdwarves to not shoot at viable targets outside of the fortifications. I've had plenty of success stationing archers on top of parapets that are 5x5 internal. Once gobbos show up, the marksdwarves all crowd to the outer edge, up against the fortifications, and begin to shoot.
Same for me. I like to make 8x8 pillboxes with stockpiles for food, drink, and ammo and stick a whole squad of 10 in there. They will absolutely SPEW bolts at anything that comes near it. Never seen them do otherwise...
Honestly, the military seems hopelessly screwed up. I wouldn't mind it being so ridiculously complicated if it actually worked. Well, I wouldn't mind so much, anyway. But I wish Toady would stop adding stuff to the game and fix some of this stuff we really need.
I agree the military is painful and should be improved, but you can't blame Toady... the community voted for improving hauling and adding mine carts. And so it's what he's doing. Toady is above average quality when compared to other developers.
Once Toady provides goblins with actual sieging weapons which destroys the walls, traps and gates... then the community will start caring more about the military working better. Until this day many gamers hide behind their fortress gates to shoot enemies attacking AND/OR use cages AND/OR use danger rooms to obtain legendary soldiers thus removing any real threat from sieges... thus there's no concern for improving the military.
The good news is Toady plans to provide improved sieging for the goblins eventually... so we just need to be patient OR play other games until it arrives.
I'm sure someone will find a way to make a fort with nothing easily breakable. Or fight stuff off with military strength.
A fort is meant to keep stuff out. If stuff can get in, there's no point in having a fort.
Things will probably shift to immediate military rather than fort design. If it stops working then you may as well stop building it. Traps and walls mostly stop the peons from being an annoyance, and a military that can stop stuff from breaching will stop the stuff that traps are for too, making them pretty useless. May as well just make a village instead, it will be quicker without building stuff that will just get destroyed which means you can devote that much time to getting military up faster.
Just notice how nobody builds castles anymore in real life, and the secure structures we do have are few and far between. Now we depend on keeping enemies from ever getting there in the first place.