My theory is that because this puzzle exercises two opposing skills, it will be challenging for anyone.

If you have a good vocabulary you may find it easy to discover the words, but you'll probably find it difficult to move the tiles into place.

If you're good at spatial reasoning you may find it easy to move the tiles into place, but you'll probably find it difficult to discover the words.

It's suprisingly difficult to solve even the simplest 3x3 word square, and you'll be blown away by the extreme difficulty of a 5x5 word square.

Maybe this is why the ancient Greeks believed that the person who could solve a 10x10 word square would become immortal.

-David