There are 17 Windmills in Dying Light 2. Windmills are a parkour challenges that require you to climb to the top of the windmill. Finding and activating all Windmills unlocks the "Don Quixote" achievement / trophy.
Activating a Windmill creates a Safe Zone around it, which provides a resting place protected by UV light.
An active Windmill enables nearby Abandoned Structures to become Faction Structures. These buildings will contain merchants, as well as offer new quests, challenges and an additional source of UV light at night.
The nature of the Faction Structure depends on who controls the surrounding area. If you have not already, you must assign the zone's Facility to either the Peacekeepers or Survivors. This will determine the people and quests you'll find in it.
