SGS Rules
Structures
Structures are mostly cities (of different sizes, but that is not relevant in the current version of the game). They can hold in addition ports and airports, and they can be fortified (with a fortress value). See illustration 4.2b below for the Structure information detail window.
You can click (left) on your own structures to enter them – See Illustration 4.2c below – which will display to you which units (if any) are currently inside the structure, its port (if any) or airport (if any).