After losing my first game of Quoridor to the kid yesterday I took it with me when I went to visit my sister. Brother-in-law (BIL) is a fan of 2-player abstracts, so I showed him how to play. We played 7 games during which we both learned a lot about the game and our strategies evolved. First we started by trying to block the other player. I started fighting back against that by placing walls to prevent blocking, allowing me to make a corridor to the far side of the board. Then we realised that you could let your opponent go down a long corridor then block it, forcing them to go all the way back. Nasty. So then we realised that the defence against that was to block off the other ways that the corridor could go, i.e. make it more restrictive, so that your opponent was not allowed to block it off. I've just been playing against the kid, and I realised that when you're heading down a corridor your opponent is making for you, the best defence is to block the back end of it so your opponent may not block the front. Awesome. Every couple of games I find a new strategic aspect of this game. I love it.
BTW, BIL played against his 5yo son, and the 5yo kicked his butt. While BIL and I played Zertz the three kids had a Quoridor tournament amongst themselves. It's the most attention I've seen a game that did not require parental involvement get from the kids.