Make an obsidian shell, make sure the outer walls look something like this
O-G
where - is empty space. Collapse the obsidian walls with clever engineering, leaving a rocky foundation at the base of the fort. I have some ideas on how to collapse just the outer shell if you need them.
Yes, please. Do tell. As I understand it, undug walls can only be affected by support busting, pick flailing or engraver work, so this is really interesting me. How would you destroy this entire obsidian shell without letting water in from the outside before hand?
Just as a note, I tried pumping down oceans before... one level works fine, but when I tried to pump out the second, the game nearly froze. not quite but it was far from playable, or useable even if i could let it run all day. So you'd have to carve out the bottom of the sea if you wanted an underwater fortress by pump.
The greatest fear of all dwarves is not the endless hoards of goblins, nor the mightiest of creatures. It is the end of the universe, by their own deity-driven will to overachieve.
------- Update -------
After a successful test of the Goblin Liquidator, I have decided to construct a twin unit. This will allow me to deal with the goblins, and let in friends at the same time. Or handle twice as many goblins as before...
Sadly, we've had out first casualty today...
... despite having a war dog tied at the end of each Goblin Liquidator., a large party going on right at the only access to the stick piles, and both Goblin Liquidators being forbidden areas. Damn kids don't got no discipline, running outside after being told NOT TO!!! Little punk deserves to be fed to the demons...
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Sadly, a blacksmith has been taken by a Fey Mood. I have no anvils yet, so he's likely loose his mind before the next caravan can bring it. I've sealed him into his room, and hope the door will at least slow him down enough to sick the dogs on him.