Build Your Kingdom
Commandeers can build buildings. You need various buildings to advance your technology & military growth.
You can also build extra barracks to train new troops. You can have 3 "actively training" Barracks, plus 1 extra for each additional Castle that you control.
Try clicking on a hex next to a Commandeer. You will be presented with the opportunity to build various buildings depending on what you have already researched. Each building has their own use, many of which granting access to new and more advanced technologies.