OK, so I finally got it and even as someone who avoids spoilers at almost all costs, I have to admit there are a couple of things about that final level that I thought were a little unfair and that I might have wanted to know going into it. So, with judicious use of spoiler tags, here are the best hints I can give without just telling the solution, in case anybody wants them (in order of recommended viewing, FYI).

1. If you don't want spoilers, don't use the hint for the final cat; I used the hint after I solved it, to trigger the final achievement, and it literally just shows the solution verbatim.

2. The solution involves a tactic that some players may consider "out of scope" for the gameplay (this is what tripped me up, anyway). So, don't be afraid to "think outside the litter box" (cat pun!). That said, after having solved it I was a little annoyed with myself for not having seen it sooner, so it's not some complete moon-logic unfair solution; if you have the puzzle-solving skills to solve the rest of the game and make it to this level, then you have the skills to solve it.

3. This one won't help you with the actual solution per se, except in the sense that it may save you a bunch of time beating your head against something irrelevant. In fact, if any of the devs ever sees this post, I would love to hear any thoughts they have about this: The spinning clock is a red herring, despite the fact that it's drawn with the "puzzle here!" yellow color and is interactable. You can ignore it completely while locating all the cats in the level.

For the devs: Is there some kind of galaxy-brain additional puzzle involved with that clock, that's so well-hidden that it doesn't even look like a puzzle (since the game says you're done after you find all the obvious cats and the one hidden one)? Personally, I found it a little unfair that you use that bright yellow with 100% consistency for every single puzzle throughout the entire game, and then use it for a red herring on the final puzzle...even to the point of making the clock interactable but still completely irrelevant to the solution.

Hey Arno, I'm glad you managed to reach the secret level, not many people do. The clock is just a visual gag, it was probably a mistake to make it yellow.