The area in the upper left indicates the construction cost. If that area is empty, the structure is free and requires no resources for its construction.
Starting in Age II, some structures can be built either by paying the requisite resource(s) or, alternatively, they can be built for free if the player has built during the previous age the structure indicated on the card.
The lower right area indicates which structures (if any) may be built for free in the following age due to this card.
The lower center area indicates in which configuration – number of players – the card is used.