1 - MONUMENT: Whenever you take the Bolster action, also gain 1 popularity.
2 - MILL: Whenever you take the Produce action, the territory with the Mill may count as an extra (bonus) territory for production. The Mill will produce as if it were a worker. If there are actual workers on the Mill territory, they may also produce resources.
3 - MINE: The Mine acts as a tunnel that only you can use. You may move units to and from your Mine as if it is a tunnel (even if an opponent controls the territory the Mine is on). Unlike the other structures, this is an ongoing ability associated with all unit movement.
4 - ARMORY: Whenever you take the Trade action, also gain 1 power.