In order to harvest wood the Lumbermill must be instructed to build a Lumbering Camp at a forest. The level of the Lumbermill affects which level forests you may harvest and how many of them you may harvest from simultaneously.
Lumbermills can only be built on grass tiles. Despite there being 4 grass tiles, you can only ever build 2 Farms and a Lumbermill.
Lumbermill can build:
Lumbermill can research:
All structure and unit stats, cost to upgrade per level, and other abilities can be seen on the Official Database Entry
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