I've taken a few years. Two years and one season, I believe. It is now Summer of Year 5, left paused on the start of summer as I have seasonal autosave and pause. I was originally going to take two years, but it took an extra bit of time to test the Dwarven Market Haggler.
9 Windmills, 8 would have sufficed but I just added an extra. Technically 10 windmills. How it works:
1: Minecarts are loaded from a level above, and set to push west always immediately. The dwarf will grab a cart and push it into the hole, where they then cannot path to it and will leave it alone.
2: The rollers along the eastern edge keep the carts "primed" and a single windmill keeps them active.
3: The floodgates prevent the carts from moving.
4: The lever to the far left opens the floodgates when it's time to discuss market prices.
5: We don't know what happens, but intense diplomacy occurs and prices are debated.
6: Carts drop to the lower level where haulers pick them up and put them back onto the upper loading level, allowing for re-firing.
7: When the lever is pulled again, the merchants have gleefully agreed to donate much of their wares free of charge!
1: Sometimes when striking a large caravan, carts can get stuck on the depot floor.
2: Rarely, live traders will be pushed into the pit on the left, and may or may not be struck by the next minecart that falls down.
3: I need to tidy it up a bit, as it is there's room for wounded traders to wander out of flight path. More creative bridges and doors would be needed for very fun haggling.
And finally, the science I've learned.
Rollers will push a cart at 2 ticks per tile.
A cart striking a creature will deal one combat strike, usually non-fatal as it destroys the creature's leg but not their skull.
Rollers take 2 energy per tile.
You CANNOT build a windmill directly over a roller, it claims there is no nearby machine!
Rollers can push a cart into a wall or doorway, keeping them at "max speed" while allowing you to open the door at a later time.
Yaks are hilarious.http://dffd.wimbli.com/file.php?id=6376
You're up Nayar, you have until the end of this year and the full rest of next year (7 seasons). This should let you build some fortress infrastructure that I didn't bother with.