It seems that World 8 (my next “most likely last level”) wasn’t the final world either, even though it was a castle. It was actually the castle of goodies, not baddies, so clearly there was more to do.
I’ve got a few more power-ups now too – a double jump (finally!), and a more powerful, and faster, dash-spin-kill attack thing. Of course, the game fights back to negate some of these when it can with stronger water currents, wind, and higher platforms. As is the way with platformers!
World 10 wasn’t the final world either, so my Platform Game Senses would dictate that World 12 would be the next likely candidate, but World 11 (which I’ve reached but not started) is a giant dark castle, seemingly set in space. Double whammy! Pretty much has to be the final world now, yes?