The empire tree is a visual survey of a player's cities and armies, located on the upper-left side of the main screen. It also provides a visible indication of the number and type of units stationed within cities and armies currently under the player's control.
The screenshot above is an example of the empire tree. In this case, it reveals a sovereign and two cities. Both the sovereign and the first city have been expanded to show further detail.
Expanding the sovereign, or any other overland army, reveals the specific units grouped together within that army. Expanding a city reveals all garrisoned units as well as buildings already constructed within that city.
The second city has been collapsed but displays two red and nine blue dots. These dots represent the number of units and buildings existing inside the city—red dots indicate units and blue dots indicate buildings.
The empire tree allows quick access to cities and roaming armies anywhere on the map. Clicking an army on the empire tree zooms the view to that army, and selects it as well. This process also works for quickly selecting and zooming to a city.