By finding the glowing pillar and selecting it you will generate a field that lifts the tumbled stones in the centre of the obelisk to form a portal. After a while, a minion will step from the portal, look around, and then vanish to do your bidding. One unit of minions count for one unit of any other resource, except for metal bars and precious metal bars, for which three units of minions count for one unit of those, so each minion created helps towards your citadel's maintenance.