remember that wagons won't path over traps, so if you want wagons at your depot you near a clear shot to it. three tiers of traps is almost certainly overkill anyway; invaders generally try to take the shortest path, so the outermost tiers are unlikely to get triggered very often. and maintenance of a big trap hallway that far away from your fort entrance will be seriously risky; your mechanics will be hauling rocks and cages for so long after any given attack that they're liable to get killed by the subsequent wave.
i almost never try to secure any portion of the outdoors- once you've cut down the trees, there aren't very many useful things that dwarves can do outside that they couldn't accomplish just as easily underground. dwarves and pets pathing around big open spaces is also a framerate killer, or so i've been told. Flying creatures like rocs and invaders with flying mounts will go right over those walls, too.
my recommendation would be to move everything underground and get your trade depot as close as possible to your main staircase to save on hauling. a barracks full of angry dwarves with sharp thingies at your entrance is probably a better defense than a long trap hallway. stonefall traps are a nuisance because they do lousy damage and have to be reset every time they're triggered, which usually means your mechanics will get tied up hauling heavy rocks. if you are going to use traps, consider using crossbow weapons traps loaded with metal bolts- they bring major hurt and don't require a lot of attention between attacks. cage and ammo stockpiles near your trap hallways are a timesaver. i usually chain dogs in or near my entrance, below ground where marksmen can't shoot at them.