The Lumber Yard is an incredibly useful resource in the game. Instead of having to worry about the finite number of trees available in the game, a lumber yard will forever produce logs for you, for only the cost of a few royal coin.
To find out more about logs specifically, such as their pricing and the recipes in which they're used, check out the page about logs.
Available at level:
First: 200? royal coin, 30 crowns thereafter
Time to build after purchase:
Maximum number per kingdom: