F F F
f F f
ccccFTTTTTFcccc
TTTTT
ccccFTTTTTFcccc
TTTTT
ccccFTTTTTFcccc
FFFFF
F = Floodgate
f = fortification
T = Trade Depot
c= constructed cage holding goblins/nightwings/harpies/ogres/general nasty things
Open spaces between the rows of constructed cages are unmined areas.
Requires:
Trade Depot, 1, underground
Flood gate, 15
Cages, 24
Nasty critters, 24
Fortifications, 2
Mechanisms, 81 (3 for the levers, 78 for the double linking from item to levers of 15 floodgates and 24 cages)
Levers, 3
Elves, Too many for comfort/More than zero
A good sense of humor, 1
Start with the floodgate between the fortifications and the five floodgates at the bottom of the trade depot open, the six on the perimeter closed, the three in at the top closed. Wait for the elves to come in, all smiles, sunshine, and rainbows- both of you. Wait for them to unload their trade goods entirely, give it a few days (in-game minutes). NOW! Pull the first lever (Lever 1) to drop the five floodgates at the bottom and the one floodgate between the fortifications. Either wait a while, imagine their unsettlement, their fear, their panic... or go straight to lever two (Lever 2), which opens all the floodgates and the cages. Hijinks ensue!
Once the elves are annihilated, go and pull lever three, which will raise the three floodgates at the top. Station two marksdwarves at the fortifications, and wait at leisure until they have annihilated the remaining beasties. Once that's over, pull the first lever again. Gather the goods, relink and reload the cages with more nasties, haul the corpses, take their clothing from their still-warm backs, and have a good laugh about these interlopers who think they can boss you around, then just walk into YOUR fortress and overcharge you for inferior goods and the occasional violent animal... in a legendary dining room full of tables and chairs made of wood.
For bonus points, mix in any nasty animals they've brought for the next batch.
And that's how you take care of wood demand.
Post Script: They count all logs on the map. See my abovementioned solution to fix your problem, I. E. you're SCREWED either way because of these picky sonsofducks.