one dwarf with a pick, another with an axe. with that and nothing else, you can have a great fortress. the number you have is simply a question of speed an efficiency. everyone starts with 3 logs in their wagon, and the two animals that pulled their wagon(which rarely match). most of us are picky about our embark points. we know the true value of sand and water, magma and vegetation. some of us may even risk the one-horned death to get closer to the sunberries. others await the fickle elves trade caravans.
so, a herbalist gathers a seed. a planter builds a field and plants a seed, harvests it, process it in his workshop, then takes the seeds to repeat the process. the loom takes the thread and a third dwarf weaves cloth. a fourth dwarf in a third shop makes the cloth into a bag. a 5th dwarf mined stone with his pick, and channeled the magma, that a 6th dwarf has a glass furnace to work. the sand goes in the bag. a 7th dwarf cut the wood, an 8th dwarf burned the wood in the 5th shop, while the 9th dwarf in the 6th shop is a carpenter at work. a 10th dwarf takes a bucket from the carpenter, and now the ashery is our 7th shop. ash to potash to lye. at the 8th shop our 11th dwarf makes pearlash from potash in the kiln. cut back to the glassmaker, and clear glass vials are ready. butcher and cook bring us to 13 dwarves, as we turn kittens into tallow. and the 14th dwarf is lucky, for he is the soaper in the 11th workshop.
So, our 14th dwarf uses the labor of the fortress, and turns kittens, wood, and sand into SOAP!
now our 15th dwarf is a mason, and he takes that soap, and makes some fine roads across the clearcut sandy landscape. at *25 per bar, that soap makes some fine roads. better than stone, better than glass. and more plentiful than most metals, at least until the next goblin siege is melted down.
soap is our best rube goldberg craft. it takes the most work, and requires a more specific map to be used at all. 14 dwarves in 11 workshops, and thats ignoring the tanner/leatherworker that benefit from our source of tallow.
did i have a point? perhaps its somewhere under the idea of a soapy megaproject. but you need a breeding pair of animals, an axe, a pick, and a lot of dwarves to work. the 'soap' challenge is an interesting one. you have to have sand and wood, magma helps, and you need a lot of stuff to work with. but whats the minimum for a soapy challenge? you can work without a pick, but you need something fire-safe to get things started. but soap dosent DO anything, you can only build with it, and stone works just as well for most things. soap needs the most resources of anything to make- its a challenge just to get a decent stockpile of soap in the first place! well, you dont need an anvil really, but you want one anyway. why not?
in the end, i simply wonder- what is the deadliest soap? is it the dread Elephant soap of boatmurdered? or do we have something more deadly in its soapy peril? Dragon soap perhaps? but dragons are rare things, you will never build a 20 z-level tower of dragon soap without cheating. how often does any fort see one dragon, much less capture a mating pair?