Level 18

The arrow points to a keyhole on a box. I've blown up, hammered, unchained, and bombed every coin, gold square, wooden box, but there is no hidden key anywhere. There is no hint that I could find. So I lost my lives and my gold many times, hoping that if I try again, I will figure it out. What am I doing wrong? The game was so much fun before Level 18. It won't let me skip, so I cannot advance. Can you help me?

Welcome to the pond!

Someone else asked the same question before and the only thing I can think of is that you missed a HOS or puzzle. Check the sidebar at the left of the screen for available things to do. Keys often appear in the puzzles, and in order to get to many of the puzzles you need to be doing all the HOS too.

If it's the level where you get a HOS asking you to find the stomper box prototype, you need to open two boxes in the HOS (can't quite remember how it works now, but you either have to find keys for the two locked boxes in the HOS or you just click upon those 2 boxes, or it's some combination of that to get the stomper box prototype) and the scene will then finish. Your missing M3 key is probably in there.