Carpenters are special workers that assist you in constructing or upgrading your Haven's buildings. Each additional Carpenter adds an extra construction or upgrade queue to your Haven, allowing you to upgrade or construct multiple buildings simultaneously.
To purchase a Carpenter place your cursor over the numbers above the "Construction" button, click "Purchase Carpenter", view the information about the price in the pop-up window, and click "Purchase".
NOTE: The number of Carpenters you may purchase is limited. Once purchased, they cannot be removed from your Haven.