There are a few versions of Windmill Solitaire of varying difficulty.
This is a relatively lax version. You win when thirteen cards (from King to Ace) are placed on each of the four Foundation piles on the corners.
In the Wiki, these are referred to as the secondary Foundations.
This is the version of Windmill Solitaire that I prefer - as it is possible to win fairly frequently.
Two packs of cards are required.
The goal is to move thirteen cards to each one of the corner Foundations.
There are two types of Foundations in Windmill Solitaire.
The Centre Foundation (positioned in the middle of the screen) and four Corner Foundations.
The Centre Foundation is built up in ascending order from Ace to King. This can be wrapped up to four times (and needs to be in other versions of Windmill Solitaire).
The Centre Foundation starts with an Ace. This can not be moved.
The top card (other than the Ace) can be moved and placed on another Foundation.
Each of the Corner Foundations must start with a King.
These Foundations descend from King to Ace. The top card may be moved.
The Tableau consists of eight columns. Each can contain one card only.
The Tableau cards are placed on the Foundations as required.
Empty Tableau spaces are filled automatically from the Stock (or Waste).
One pass only through the Stock is permitted.