The steel, btw, doesn't have to break when the vampire hits it. The vampire could just claw at it and tunnel through.
Which is why you make the walls out of caesium with a layer of tungsten carbide on the outside. Sure, they might be able to claw all the way through the metal... and reach the layer of water that the cage is submerged in.
This is just another example of why vampires are not nearly as scary as engineering is. They are a lot more sexy, but the reason even they don't control each other using modern science is because it isn't as sexy. Sure, Alec can anaethatise folks and they can all tear things to shreds in their immediate vicinity. Edward can incapacitate all girls and gay guys in the immediate vicinity with sultry looks and incidentally read minds and run fast. All terribly sexy and dangerous. All pale in comparison to what you can do with an ICBM or a surgical strike with napalm bombers.
Bella was playing for keeps (and not being a protagonist in an engaging fictional piece) she could have wiped the Volturi from the face of the earth in less time than it took to turn a bunch of native Americans into big sexy-but-only-moderately-dangerous canines. She could use the skills of the Cullens to infiltrate a suitable military base and use military resources to level the Volturi headquarters.
There would be difficulties to overcome and research to be done. They would need to find a way to stop Edward being shot down by fighter aircraft while doing his bombing run or a way to navigate all the security protocols protecting nuclear missile launches. These may be real challenges. But they are all challenges relating to overcoming those with the real relevant power: human military organisations.
The hard part, of course, is finding a way to replace the Volturi, sans the evil. It would take human-equivalent decades of time to develop technology for suitable non-lethal force against vampires. Then more time to arrange for suitable prisons. And the socio-political difficulties in both creating a government and in dealing with a bunch of xenophobic humans seem to be very nearly insurmountable. (So the Volturi are doing a more important public service than any other government.)
and reach the layer of water that the cage is submerged in.
And this part would do what, exactly?
In the vein of the Harry Potter and the Methods of Rationality discussion threads this is the place to discuss anything relating to Alicorn's Twilight fanfic Luminosity. The fanfic is also archived on Alicorn's own website <strike>(warning: white text on black background)</strike>.
Previous discussion is hidden so deeply within the first Methods of Rationality thread that it's difficult to find even if you already know it exists.
Similar to how Eliezer's fanfic popularizes material from his sequences Alicorn is using the insights from her Luminosity sequence.
Spoilers for the fanfic itself as well as the original novels need and should not be hidden, but spoiler protection still applies for any other works of fiction, except for Harry Potter and the Methods of Rationality chapters more than a week old so we can freely discuss similarities and differences.
EDIT: Post-ginormous-spoiler discussion should go to the second thread. (If you have any doubt on whether you have reached the spoiler in question you have not.)