What important lesson have you learned about organizing a fortress?
Say what the lesson is, and how you learned it.
It can be anything from little mistakes, to fortress disasters.
I have two, both involving water. The first isn't bad, I accidentally flooded my archery range when I channeled a rover into a diagonal space in the wall. Oops.
Lesson: Always check to confirm your designations aren't going to affect anything besides what you intend them to.
Now this one destroyed my latest fort, three times. I was trying to build a very, very big underground fishing zone. About 25x25 tiles of water and walkways for the fishermen. Anyway, the first time I forgot to close the floodgate leading to the indoor lake, and flooded almost all of my living quarters. The second time, while draining the lake into a bigass storage zone underneath it, so I could haul out the stone and make adjustments, I accidentally opened the floodgate leading to the lake, flooding my fortesss again.
Lesson: Always remember the current projects, and check up on things often.
The last time, the one that made me just give up, was a combination of old floodwater filling the lower living quarters, and a few silly mistakes which made draining the water a very big problem.
Lesson learned; think things through, or die.
Add your own!