My current human fortress makes use of 'classes' of people (in the social sense, not the D&D sense).
I use burrows to allow certain classes of people in certain areas, the safest and nicest areas only allow the upper classes, wheras the workers live near their place of work, and I have the lowest rung of society being able to go anywhere but for strictly hauling only purposes. (of course I personaly pick who is suitable for what class, legendary cheesemakers go on the bottom rung, wheras my doctors and smiths go at the top. this also helps keep your important dwarves from ever being near the caverns of outside, so they will never be caught by ambushes or wildlife like the peasants)
The old camp kinda does this, people are only allowed into the castle with given permition, so you could use that technique to recreate that real sense of seperation the old camp had, dont let your important mages mingle with the filthy workers!
I suggest making use of those hordes of migrants and putting them within work groups, some types of work dont need high skills all that much, woodcutting or 'clearance mining' (mining for the sake of landscape clearing, rather than resource gaining) or masonry/carpentry so long as its for building walls. -you can use this to get those massive walls built pretty fast providing you make good use of stockpiles.
A good tip I picked up- assign your 'useless slave' caste to all use masonry and carpentry BUT assign your workshops to only allow your handpicked skilled craftsmen. that way your hordes will quickly assemble city walls, but will not be taking workshops away from your skilled workers.
I'd suggest building the outer wall first, get a mass of woodsmen and carpenters to get hasty pallasade up, then you can take your time with the castle proper. also note that Humans dont REQUIRE alchahol, they can exist off it pretty well without the side effects you would expect, but they wont get all grumpy like dwarves do. I suggest having a good supply of water from wells, with booze being an optional extra when you dont need the manpower elsewhere.
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