Well, its not 100%, but this works pretty well:
###################
PPPPPPPPPPPPPPPPPPP
TTTTTTTTTTTTTTTTTTT
TTTTTTTTTTTTTTTTTTT
TTTTTTTTTTTTTTTTTTT
PPPPPPPPPPPPPPPPPPP
###################
#: Wall
P: Pit, 10ish Z-Levels Deep
T: Wood training spear traps.
Put this as the entrance of your fortress(with a ceiling over it in case of flyers). The Wood spear traps don't do much for damage, but enemies will dodge them and fall into the pits of death. Since Caravans are friendly, they won't trigger the traps and will make it through just fine.
This style of trapped entrance works great in an evil region with zombiefication. Anything that falls down into the corpse pit and survives gets torn up by all who preceded them.
Forcing the enemies to dodge into a gutter on either side of your entrance... I like that! The only problems I foresee with it are that, aside from the fact that the wooden spear traps would also hit your caravans, there is also the big issue that, after awhile, the zombified creatures and body parts would start to build up in your gutters and eventually overthrow your fortress by way of framerate. May I suggest you fill the gutters with magma? Or, if you're feeling really bold, you could hollow out the entire area underneath your trapped path, and then create a deep gap between that area, where your undead will be rendered harmless by the distance, and another, on which you can station your marksdwarves, or train up your hunters. Use bone or wooden bolts, and you'll have an endless supply of practice, while yet keeping your framerate low. Once you've built up too much a surplus of undead, you can flood that area with magma from the sides, and it will all drain helplessly into the gap between that area and your marksdwarves.
FOOOO=MMMMM
XXXXXX=MMMMM
XXXXXX=MMMMM
XXXXXX=MMMMM
FOOOO=MMMMM
X = Floor, with trapped area directly above.
O = Floor, with the pitfall directly above.
F = Floodgate, built into the side-wall, behind which lies a store of magma.
= = A very deep gap, at the bottom of which should be a series of pumps, to re-circulate the magma back up to the magma-stores. Alternatively, you could have some fun with it and drain it into your caverns, to kill whatever nasties might be hiding down there.
M = Floor, on which you should station your marksdwarves.
Creativity is fun.