The Alder Windmill is located in downtown Trinity, east of Saint Joseph Hospital. You’ll probably notice it when Hakon first gives you the binoculars. But while it’s the first windmill you’ll see, it’s not necessarily the easiest to activate.
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The first part is quite easy. Climb onto the wooden platform on the south side of the windmill. Now turn north so you’re facing the large concrete block that goes up and down on two cables. Even when the cinder block is at the bottom of the cables, it feels like it’s too far away to make the jump, and you might not get it on the first few tries. But if you time and aim correctly, and hold the jump button while jumping, Aiden will make an unusually long jump and you’ll hit the concrete block.
Stand on the block and wait for it to reach the top of the cables, then jump onto the wooden platform on the opposite side. Go around the corner and then up the ladder to the platform on top of the windmill, where you can activate its power box.
Activating the Alder Windmill gives you 500 Parkour XP, unlocks trading posts to the northeast and southwest of the windmill, as well as a safe zone, UV spot, and stash.