The lack of civilisation in the Mushroom Kingdom can represent the lonely world we live in. And the fact that the only civilians, the Toads, are hiding in castles might symbolise that we're all afraid. The fact that the Princess is in another castle might be a reflection on the human attitude that no matter how hard you strive, you'll never reach you're goal (until 8-4). The warp pipes might be an indication to taking shortcuts in life, but in doing this you miss out on what life has to offer (like 1-ups).
