Thanks! The collection is actually based on data from computer search, i.e. the deepest states possible for each possible wall configuration. The set was pruned, hand-tested, re-ordered and pruned again, to try to eliminate repetition and add a sense of progression. Grids that were discarded along the way were sometimes recycled as hand-crafted tutorial puzzles, to demonstrate a particular feature. Inevitably many puzzles have traps where you can get stuck but these were carefully positioned so, in the early ones at least, it was very obvious what states you had to avoid. It was a fun process refining the set. We also looked at the 4x4 data - but I think "A Bit Much" would be the right phrase there. The 3x3 grid was definitely the sweet spot.