Well, these problems are strange to me. Not because there isn't a problem with dodging - there is one. It's strange because enemies need very high level marksmen to shoot through fortification (unless they are standing next to it, which in case of towers is irrelevant). I rarely see enemy archers above level 5, usually they are in range 2-5, or even without skill, like zombies. And I have a fort which is besieged at least twice yearly, but sometimes even every season, by elves, humans, goblins and assorted products of necromancy.
However, I've lost a dwarf, my legendary doctor who was also a legendary marksman, to the above mentioned bug. He somehow jumped out of tower, and landed under it. My towers are not high (higher tower means lower range), so he was only temporarily stunned, and thanks to his skills he almost survived till the soldiers came to defence, but even the best soldier won't do much against a pile of 15 goblins, all of them in a single tile. I suspect a dodge from a dog caused it, because dogs and other critters sometimes charge when crowded, and this may trigger a dodge. But not arrows - arrows need better archers than average gobbos have.
I use the standard and necessary precautions so dwarves don't charge, and the tower cannot be climbed - the edge is protruding, the fortifications have a roof above the standing place for dwarves. The place is only one tile wide (which may have contributed to the death of my medic) to let dwarves under level 10 to shoot through them. Though I use such training regime, that the dwarves get level 10 after about 2 weeks.
__ __
F F
W W
W W <-enemies come at this level
WW WWW
WW barracks at level -1 WWW
W = wall
F = fortification
_ = floor (one above fortification is a part of original wall, which was then carved into fortification)
Dwarves are standing next to F
My tower is on the beach, with narrow passage between it and the ocean. I named the beach "Omaha", because no enemy comes through it unscathed. In fact many stay here as corpses for months. The marksdwarves are super effective. Not as much as melee of the same level, but they are safe while doing so, and if I let the melee dwarves out, the crossbowmen give an outstanding support. Every enemy resembles a pincushion.