The 1st time i see that.
I had a fort in which i made it as a personal challenge to not build any traps, only walls and doors and military to resist goblin sieges.
Managed surprisingly to defeat 3 of them and with some casualties in the dwarf side each time, but not enough to decimate my troops, as i was lucky enough that the game actually provided competent warriors and marksdwarves for once.
Then the 4th siege arrived, near to 100 enemies, a big one, and this time a lot of Trolls, a very lot more than in the previous siege.
After the Troll legion destroyed my gates, and invaded the halls, the whole goblin army and their pets followed, and then it was a bloodbath, despite the brave resistance, the whole military was annihilated, mostly destroyed by the very numerous powerfull Trolls despite some of the dwarven soldiers had some very high skills in their weaponry they were simply no match even with the support of the marksdwarves.
As it was going to end with a massive carnage, i decided to just browse the internet while letting the game gets to its end.
Then after a while, i checked to see if it was ended and was surprised to see the game paused due to "a migrant arrived"
I thought, poor guy, probably going to die in seconds, then i noticed the "SIEGE" warning on top of the screen wasn't there anymore, and checking in the fortress, that was full of dwarven corpses all around, there was no more invaders anywhere.
I wondered how the game didn't ended as the fortress was gone, and noticed in my dwarf list that there was in fact a survivor from the siege, a fisherdwarf that at some point of the log was interrupted by a troll (all the dwarves were inside the fortress when the Goblin invasion crushed the defensive perimeter).
Then he got a lot of tantrum and calmed down several time, became Mayor (as he was the only dwarf) too.
Looking at his status he has the left hand broken and has a teeth gone.
He is a competent hammerdwarf and has a kill (a dwarf that went berserk at some point of the fortress life) and i highly doubt he was able to resist the army of Trolls coming there or the goblins and various monster pets following.
But he's always alive and he is in the fortress alternating between tantrum and calming down, while all around him every dwarf is dead.
I really can't understand how it can be possible for a dwarf to have survived the massive onslaught.
Now that he has been joined by a single migrant, i wonder if i should prepare for the next and probably last goblin siege