As soon as I post I understand what it is that I was talking about - that is pretty normal for me. I suppose the whole point of riddles is that they are not like good crossword clues .
Is the very word 'riddle' meant to be a sort of clue ?
The the two things puzzling me most as I contemplate the end game is that I expected to see a U in every quadrant; and why the double lines have a two-cell break between quadrants, are they a passage to move from one to another?
You were perhaps expecting a U to emerge from each quadrant into its external square, lumen - but only one of the 4 external squares is required and the U passes through the grid to that point (you've got a good idea how).
Yes thanks, I'm getting there, I have three of the riddle solutions now, just need to connect through Top Left - but my TL riddle is gibberish with ten words, so think I may have gone wrong!