This is how I would do it:
Yellow is the channeled entrance 3x3.
Blue are floor grates linked to a lever. If pulled they open up to a fall of 15+ z levels ending in pointing spears (this kills or seriously cripples even a bronze colussus). Seeing goblins drop several z-levels and get crushed on a rock floor is really something.
Green are cage traps (these are op as fuck, you got a war dragon attacking? A cage trap will stop him, thats one stone and one piece of wood. Doesnt work on some legendary creatures like forgotten beasts though).
Pink are raised bridges, ready to crush whatever is beneath them. You could change these for something else like serrated blades traps. Those are pretty strong, although they can jam and when the dwarves release the trap it cuts them in two. They are also bad against zombies (creates huge amounts of zombie limbs walking around), although after the fall and cage traps there probably arent many zombies left.
The dark yellow spots are where dogs are chained, so that you can see those sneaking goblins and kobolds, which otherwise walk over your traps.
The red is a 3x wide area free of traps where the caravans will travel to get to the depot (remember to cut down mushroom trees), if you make this one particularly long and have the trap area suffienctly wide then the invading creatures are bound to take the trap road. If you still feel unsafe in that red zone then you can mine 1 step to each side and put cage traps in the corners, that almost guarantees to catch those rare monsters running in there. Perhaps in the one in a thousand event when a dwarf actually runs back in the hole through that zone while being chased (I have yet to see a dwarf run back into the fort when attacked, the idiots just run in a random direction as fast as they can.)
Really though, having only traps and no military leaves you with less flexibility and ability to save dwarves outside of your fort, and it also reduces the chance of survival if something goes wrong. Even having a moderately trained squad with copper armor/weapons can make a huge diffrence.
The weakness with these traps are that if you open the grates then the only pathable way in is though the red zone, which means they will take that way. This can be prevented with a bridge in the red zone which can be raised to create an instant wall, few monsters can destroy. Another flaw is if you somehow get a flying forgotten beast outside. It will fly over the grate holes, ignore all your traps (since it got trapavoid) and ignore the bridges falling on its head (some monsters are immune to it, probably to prevent abuse). Really though, if you are avoiding a military the only sure way of being safe is walling yourself in and thats just lame imo.