OK, so with the game started (and completed) in just a few hours yesterday, I set about opening up all the pathways I’d missed – mainly with the aim of opening up Worlds 4 and 7.
It seems some of the pathways I missed first run through were, in fact, the “normal” routes, and I’d accidently taken secret exits and therefore detoured. Specifically, on World 6 I seem to have missed out almost the entire set of levels. So, plenty to do today then!
Some of the remaining secret paths were a right pain to find, and some required use of the Blue Koopa Shell power-up – a somewhat rare item, it seems. Cue loads of trying and retrying to get it from a random powerup item box in a Ghost House…
Anyway, with a few more hours jammed into the game, I think I’ve opened up all the secret paths. All of the levels within each world are now completed, but I haven’t unlocked some gates which lead only to Mushroom Houses. I still can’t figure out how to reach World 4 or World 7, however. Bah.