As it turns out, doors do NOT necessarily keep goblins out of the above-ground portion of my fort. Funny thing about the centralized stairway design that I favour: it allows a goblin siege to very quickly wreak havoc in a fort.
Got lucky, though--my military, consisting of nothing but marksdwarves, was able to soften up the invaders. Although there were numerous casualties, my miners actually broke the siege by walloping goblins with their picks.
So, no problem, right? I started to inter the dead, building a warren of catacombs and tombs. Only about 14 dwarves were killed, leaving around 70 or so.
That's when winter hit. It was a temperate climate so the ponds and river froze (already had problems with some dwarves breaking through the ice in the spring). I had not yet built a functioning cistern (although it was almost ready to go). Of course, I could not fill the cistern while the river was frozen.
Around mid-winter, I has a long series of "Urist McDwarf cancels give water: no water source". I assumed there were some dwarves who were injured and took to their beds. I was pleased with how diligent the dwarves were in caring for the injured.
That's when the great die-off started. One after another, my little dwarves succumbed to thirst. Checking the stocks screen, I realized--my booze stocks were gone. i've always had some difficulty in managing booze, but the presence of water has usually been an adequate cushion.
By the time I was able to rebuild my booze stockpile, I was down to five remaining dwarves, one of whom, thank God, was my mayor and broker.
Come the spring, migrants arrives. Putting them to work on the clean up, particularly the dessicated corpses lying everywhere, the elves arrived. At the same time, the ice started to break up, allowing me to build that cistern.
That's when the goblin ambush struck. In the race for survival, I had not replaced the doors with something more secure. No problem, right? The elves were there. Elven arrows would discourage the goblins. Right? Right?
The goblins raced inside the fortress, hunting down every dwarf they could find.
The last dwarf standing was my mayor. Chased into a dead end tunnel, with five goblins in hot pursuit, he was quickly surrounded. Thus ended my fortress.
The elves never moved from the Trade Depot.