You're not alone if you're struggling to solve the windmill puzzle in A Plague Tale: Requiem. You won't find this puzzle until you reach Chapter 9, by which point you should've opened the way for Arnaud and perhaps saved the herbalist. In contrast to those examples, you might be pleased to learn that this is a more traditional—and optional—puzzle.
You should be feeling right at home in rat-infested medieval France by now, but that doesn't mean there aren't things left to discover. So if you're ready to solve the windmill puzzle in A Plague Tale: Requiem, here's what you need to do.
A Plague Tale: Requiem windmill puzzle solution
Once you reach the open area in Chapter 9, you will find four windmills and an underground map in a cave that gives you clues to the puzzle. Basically, you'll need to ensure the correct windmills are turning to nab a unique bracer for Amicia. The windmills are numbered with Roman numerals and the solution to the puzzle is to make sure I and II are off while III and IV are turning.
You can walk straight in through the door of the first windmill and interact with the mechanism at the top to stop it from spinning. The second windmill requires you to pull away some boxes blocking the door before climbing up and squeezing through the gab to get inside. Skip over the third one as it's already turning, then crawl under the wooden cart at the fourth windmill to get to the ladder. From here, you can shoot the lock on the inside of the door through the window to access the mechanism inside.
If you've done it all correctly, the ground will shake and Hugo will comment that it feels like something moved. Now you need to locate the underground cave.
Look for the stone archways nearby and head through them down the slope. At the bottom, turn right and look for the top of the ladder sticking out of the hole. Make your way down and follow the tunnel past the underground map until you reach an open area, then follow the wooden walkway around to the right. You'll find some goodies in chests here, along with a bracer that makes Amicia recover from enemy attacks faster.