Playtesting should be a key part of your design process. Done correctly, playtesting will transform your game design into something powerful.
This article covers the guidelines for puzzle design that I used in Puzzledorf and my answer to the question, "What makes a good puzzle?" It's not intended to be the only answer, but instead is a guide through my journey about what worked for me.
This article talks about the difficulty level of Puzzledorf, how it changes over time, and how that is used both to engage and retain players to help them finish the game.
Teaching people how to play your game is a critical component of game design - and yet a lot of players hate tutorials. So how to teach people without boring them? Better still, how to provide a tutorial that players don't think of as a tutorial? This article discusses the subject through the lens of Puzzledorf.