Another suggestion for the Fortress of Solitude this time is that it doesn't have
to be a huge ice-palace.
I suggested in another thread once that the Kryptonians may have come to Earth thousands of years ago (perhaps during the ice age) and built the Fortress to blend in with the surroundings.
I always felt a disguised Fortress somewhere in the Arctic would be perfect. It can't be seen by humans, it can't be reached (at least, not easily) by humans and the technology in the fortress can render our satellites etc useless so they can't see it either.
And what would be the perfect disguise for the Fortress in the arctic? An iceberg. Small on top but with abundant space beneath it, no human would ever suspect it's anything else.