I had an idea not too long ago about a fortress built as a succession of 'cells' that could be sealed off from one another with a few levers. Essentially it was a series of globes or towers submerged in magma (or even just free standing could work, if you were lazy) with walkways running between them. A few diagrams-
===BBB=== F0
+D+++++D+
+D+++++D+
+=+++++=+
+D+++++D+
+D+++++D+
===BBB===
===^=^===F1
+++|+|=++
+++_+_=++
=========
++=_+_+++
++=|+|+++
===V=V===
F0 is the walkway, with D for doors and B for bridges (raised preventing flow, and lowered to allow flow). The above floor is engineering, which contains pumps to clear out the walkway to re-allow access. Note that the design has the two segments separated to ensure perfect seals. Both ends of the segment have to be cooperating to open a walkway: north side controls north pumps/bridge, south side controls south pumps/bridge. If either bridge is open, then the walkway remains flooded. Since magma doesn't react normally to pressure, substituting water would require flooding the tunnel through a diagonal link (to reset pressure), which means slow flooding, or using another set of pumps outside to pump water in to equalize pressure.
Put it on hold when the new version came out, but I might pick it back up if the pathfinding bugs don't get a fix in the near future. It's aggravating to have to constantly save and load just to get my miners to mine.