I hadn’t made any Cell Wall Transport System puzzles in a long time, but when the tenth anniversary of Knossos Games and Imagine magazine came around, I knew I wanted one of these puzzles in the line-up. It was an important puzzle set for me – one of the first I created from a set of rules I had “discovered” (instead of taking an example puzzle someone else had made and modifying it in some way). It also won the “what’s your favorite puzzle” contest I held on my website. In order to come up with some new ideas, I turned to an old, unused puzzle.

I used an idea that appeared in the largest of the Cell Wall Transport System puzzles I had created back in 1996 (the one which wasn’t very good, mentioned in the comments for Puzzle 1). I realized early on that I could force a finish compartment by making it attached only by symports and having a receptor inside. This way, you’d have to bring an object with you to get in, you’d leave it in the receptor, and then you wouldn’t have an object to take with you to get out. You could actually create a chain of these, making the first one the finish line. So I tinkered with different ways to implement this and created both Puzzles 5 and 6 using this concept. Puzzle 5 was originally going to be published first, but I discovered a problem with that puzzle right when my submission was due, and Puzzle 6 was ready to go, so it got published instead.