So.
A wall is nice, a natural wall is better, and some dwarves have used completely isolated pods made with natural stone and cast in obsidian...
but the biggest long-term threats to a fort are boredom and fps. The former is inevitable. The latter?
well. One can try magma. but if there happens to be a lot of magma-safe junk lying around, or worse, nearly magma safe, then this will make the fps unbearably bad for days.
So. get rid of the world-clutter via traders.
Which means no isolated pods. But then I was inspired by some shenanigance involving a forgotten beast...during which not a single dwarf was lost. because the beast decided to admire a door instead of destroying it. It's still there. (pics when i get home. maybe.).
There's still a connection to where the beast resides, but it now includes 1 of each seige engine and lots of walls. plus enough doors to slow it down for long enough to dig out a brand new fortress.
This was all a bit random...but I've decided I shall rely on the phenomena from now on...
I shall indeed expand above ground; but not with a single mega project, oh no, i'm going to use lots and lots of these:
***++***
*XXXXXX*
*XBBBFX*
+XBBBFX+
+XBBBFX+
*XSSsbx*
*XXXXXX*
***++***
*=wall,+=door, X, x=trap, B=building, b=bed, F=farm, S=stockpile, possibly of the quantum variety.
Are they designed to be independent? no. They're designed to be almost-independent while also being reasonably simple. And able to trade with caravans...so that if a dwarf IS stranded in one, I can at least keep them from starving/dying of thirst as long as I notice.
Meanwhile, doors can be slammed in the face of enemy trapavoid+buildingdestroyers; enemies that are only one or the other will be stopped by a combination of cages and humble stonefall traps.
And when a building destroyer+trap avoider arrives? Well. it'll have so many walls and doors to choose between, it'll have to take its time to get to the dwarves, allowing time to not only get dwarves to safety, but to finally train up a military.
it'll be REALLY nice when I finally get around to playing with magma...It'll stay in one cell as long as i only dump a little bit at a time.