I'm on a continent that's separate from the mainland inhabited by only an elf kingdom called Tise Ewe, which I've had cordial relations with as I have actually made a concerted effort to avoid murdering them and stealing their stuff. I went for about ten or twelve years without anything, and then some kobolds appeared and were beaten to death summarily. Around a decade after their intermittent attacks, goblins began to appear to try to snatch. None of them succeeded and all of them died. I devised some traps to ensure they never got through.
Then they ambushed me a moment or two ago.
The sick bastards appeared as I was trading with the elves. I would've been able to trade enough junk to acquire lumber to build more beds and barrels for wine. They were mostly crossbowmen with two melee. The crossbowmen killed quite a few of my civilians(my definition of quite a few is eight or nine) before they could get indoors. The melee got into my ungated(I had no need and believed ambushes/sieges were impossible if there were no nearby dark forts to send them from, and you don't need gates for snatchers) surface fort and killed an elven yak and nearly an elf. Then my valiant military showed up and my commander wrestled with him and threw him into my 7/7 deep water canal for my well, where he drowned. My dwarves began to trade crossbow fire with the goblins, who were terrible shots and fell fairly quickly or outright ran. The other melee was unfortunate to come across my militia commander when she was angry and was beaten senseless and thrown into the river, where the carp had at him. It was amusing to watch.
Anyways, I'm certain that no dark fortresses existed when I embarked on this continent. Can ambushes appear anywhere that your edges of the map aren't totally water, magma or sheer cliff? Or is something else afoot? Is Tise Ewe fighting a war with the goblins I knew nothing of, and have they established a beach head? I fear the worst, as this is my first contact with goblin invaders.