There are some Windmills all over Dereth, mostly in the Northern and Middle Osteth areas that have mostly an Aluvian presence. They're typically there for decoration, and the scribe has not yet found a single windmill of functional value.
When one goes inside a windmill, there's a slowly-spinning shaft that makes a sound effect of a regular drumming rhythm.
Sometimes, the Windmills are useful in helping one fight against the typical fauna of the land. For example, if a swarm of creatures chase the player, they can run inside the windmill and create a "bottleneck" effect at the entrance, therefore get to fight just one creature at a time, and therefore protect themselves from being surrounded by too many at once.