Here are a few of my favorite accidental fortress designs:
All .34 days though
-I once flooded an entire level with magma due to a bad pumpstack. Now I put multiple magma safe doors between .
-Another time I dug a shaft straight down to the magmasea with my starting seven, created a nice little fort with no doors. Then I wanted access to timber (frozen ice biome), so I dug up towards the caverns.... Dug straight into an underground lake. No one died as I dug out a drain into the magmasea, but it did result in the mud fort. Made farming easy.
- Ahhh and there is Tinfingers and the tale of the Red Hand. An obsidian desert with fire clay and FUUUUNNN!!!!
Ogre once smashed a floodgate for the main entrance/flooding trap. River filled up that area, elven merchants got stuck and went insane. Created a nice lake. Created a nice entrance to the tower on the non river side, promptly got filled with ogres and elven ambushers.
But that isn't all. Tinfingers also had a nice curious underground structure and forges at the magma sea with ample surplus weaponry. Well I pierced the 3rd cavern by accident and around 400 zombies started flooding into the main staircase. My fort was split into two; so I burrowed my smiths into the forging area and forced them to mass produce arms and conscripted every dorf. Over one hundred bodies were thrown at the gap in the wall, a plug made of dwarven flesh to hold back the undead deluge. 70 souls died but the survivors were branded Veterans. I felt such pride when I saw these fellows in subsequent forts after I abandoned Tinfingers (constant Ogre/Goblin attacks and no migrants made the situation untenable.)