My first truly successful fort - food under control, population being accommodated, excess cats under control and everybody happy. The nobles were trickling in, the dungeon master had just arrived, it was time to start getting frivolous. So, I was laying out the building plans for ornate bridges over the underground chasm, carving out the rooms and setting cages for the bestiary, engraving every square inch and digging deep and greedy for surprises and fun.
As I was quick-panning around the plans, a flash of blue went past the screen.
Wait, blue? My dining room isnt blue. Go back.
Flood.
As a fun frivolity, I had built in a self-filling reflection pool in the ornate dining hall. This was my first attempt at using pressure plates for anything, but initial tests seemed to work - the pool filled, the flow was shut, everybody happy and we were assured of fresh water from the surface forever. This was one of the first things I had built, maybe in year 1. Since there had been no end of watered dwarves, I assumed it had been working without error for some time. But, apparently this was the first time the mechanism kicked in.
By the time I noticed the spill, the water was too deep to get in and flip any switches, too deep to wall off the feeding channel. "MASONS!", the cry ran out, "Seal off the dining hall!". Tense moments went by as water poured into the dining room.
"Where are the masons?!"
Sleeping.
"Oh, hell. All you dwarves, pick up mortar and brick and get busy!"
"No good, its too deep, we need to fall back!"
"Wall off the throneroom and treasury!"
Screaming dwarves were blocked in with the rising water, rotten dinner and all our fortress's jewels, but we couldnt spare a minute. Whats the status?
Water spilling down the main stairway. Dungeons, jails and crypts flooded, 3 dead in the lowest levels. Think fast.
"Seal off the stairways!"
The population was trapped on whatever level they happened to be in, as the central staircase was turned into a column of water that terminated on the surface. The flood was contained, but the central architecture was ruined. Dining room, treasury and throneroom lost. Crypts and jail lost. Apartment level: ruined as dwarves ripped down walls and tore through bedrooms to get to the staircase in time. dozens dead, the rest starving.
Emergency egress staircases were carved wherever they could be to allow the starving, trapped dwarves access to food and water on the surface, and we faced the decision of how to go about rebuilding when so much of the integral structure had been compromised and lay underwater.
In the end, I decided to hold a very important meeting about the matter.
Under a drawbridge.
Smoosh.