First off, what is the difference between a wall grate and a set of vertical bars? Is there one? Which should you use to keep (for example) lizardmen or giant crocodiles from swimming up your canal and munching on tasty dwarf around the well?
Second: I had a strange thing happen while playing and I'm trying to figure out what, exactly, transpired. The setup: I dug a tunnel to my river (the other end dropped into a cistern). I then set up a set of vertical bars, and stupidly, designated them 1 tile away from the end of the tunnel. Since the tunnel ran left-to-right, it meant that the dwarf happily walled himself in (since they apparently prefer to work from the west). Well, fine. I wanted to test a theory I had about them being able to swim out of channels anyway, and he was a fresh-off-the-wagon immigrant with no skills. So I left him there.
When everything else was ready he was still alive (thirsty, though). I sent a miner over on the z-level above and channeled out the 1 remaining tile. Of course the underground tunnel immediately filled with 7/7 water. Hrm. Looks like the peasant was going to drown -- he wasn't swimming out from above (the only open route), and the vertical bars were in the way preventing him from outrunning the flood.
Then, suddenly, he appeared on the OTHER side of the bars, sprinting as fast as his feet could carry him, and actually managed to outrun the flood and get back through the maintenance corridor before the water filled the tunnel.
So, what? Do dwarves have some sort of compressable skeleton that allows them to squeeze through bars? Can they teleport? What gives here?
(I've checked, and the bars are still there.)