The Druid Willow is available as one of the options through the Celtic Forest Selection Kit which also allows to upgrade the Druid Willow to level 2, thus improving its properties.
For more information see Set Bonuses.
The Druid Willow comes with the special set bonus. When it is adjacent to other buildings belonging to the Celtic Forest Set the properties of the building are improved.
The Druid Willow can be upgraded by Druid Willow Upgrade Kit item. To upgrade the Druid Willow, use the Druid Willow Upgrade item on the Druid Willow that is already situated in your city. Its upgraded version will then be placed directly on the same spot in your city and the production timer will not be reset. Note that the upgraded building will be from your current era, even if the old one was from one of the earlier ages.
Druid Willow will produce the following effects. Base effects will always be produced. Bonus effects are determined by how many other unique Celtic Forest Set buildings it is connected to.