Okay, so I genned a bunch of worlds and eventually found a site I was looking for. A straight section of major river, lots of limestone, and a magma pipe. The idea is to build one tower on each side of the river, walled in with the only access to each being from the other tower and from a bridge extending across the river, such that you have to cross a long bridge to get into the fort, but you can get in from either side of the river. (The bridges are to be overlooked by crossbowdwarves, obviously, and possibly have a ballista or two pointed down each.)
So, the problem...
Year one: no traders.
Year two: elves show up, orc siege.
Year three: elves show up, orc siege.
Year four: elves show up, goblin abush, orc siege.
Year five: elves show up, orc siege.
Year six: elves show up, humans show up.
In all this time, not a single dwarven caravan. Every season "your fortress has attracted no immigrants". A repeating drawbridge system and stout walls are enough to shut down the sieges, but going is slow with only my starting dwarves in charge of quarrying, building the towers, and keeping themselves alive. Plus how can I hope to rain bolts on my foes with only my starting dwarves, none of whom can be dedicated entirely to military?
So, my question: why would this happen?
Population cap is 150, the embark screen shows access to dwarves, humans, elves, orcs, and goblins. Only one dwarf civilization is listed. Legends mode.... legends mode froze when I tried to enter it, and I'm not sure what to look for there anyway. My fear is that this means my civilization is dead and I'll never get a population unless I have children or something. My hope is that as humans didn't show up until the 6th year, perhaps dwarves are taking their time about it as well?