This information may be obvious to some of you, but I only just recently noticed it, so I'll mention it.
I've noticed that the shapes of the unit icons have meaning for the type of unit (round for military, triangle for civilian), but that they also change with the state of the unit. Some examples:
1. Fortification: shield icon. The units on the left are fortified. The icon returns to a circle as they attack.
2. Water Embarkation: pentagon. Note that the embarked icon is the same for military and civilian units.
3. Icon dimming. In both the regular and strategic view, in some cases, some of the player's unit icons are dimmed. I'm not sure what this indicates. Someone suggested that it has to do with whether a unit has movement points left, and this is a good idea, but it appears to be more complicated than that. If you watch the video in which the riflemen are fighting and the ships are bombarding each other, the icons of both the selected unit and units around it flash and pulsate and switch states from dim to bright and back again. So it's not clear what these states indicate.
4. City Garrison. The icon takes on a gold border. It is also positioned to the upper left of the city bar, whereas other units that appear to be just sitting in the city have the icon to the upper right. (By the way, the red target icon below the city bar is the button to active the city's bombardment ability.)
Longsword unit on city tile but not garrisoned: icon on right instead of left (ignore arrow)
I've noticed that the shapes of the unit icons have meaning for the type of unit (round for military, triangle for civilian), but that they also change with the state of the unit. Some examples:
1. Fortification: shield icon. The units on the left are fortified. The icon returns to a circle as they attack.

2. Water Embarkation: pentagon. Note that the embarked icon is the same for military and civilian units.

3. Icon dimming. In both the regular and strategic view, in some cases, some of the player's unit icons are dimmed. I'm not sure what this indicates. Someone suggested that it has to do with whether a unit has movement points left, and this is a good idea, but it appears to be more complicated than that. If you watch the video in which the riflemen are fighting and the ships are bombarding each other, the icons of both the selected unit and units around it flash and pulsate and switch states from dim to bright and back again. So it's not clear what these states indicate.
4. City Garrison. The icon takes on a gold border. It is also positioned to the upper left of the city bar, whereas other units that appear to be just sitting in the city have the icon to the upper right. (By the way, the red target icon below the city bar is the button to active the city's bombardment ability.)

Longsword unit on city tile but not garrisoned: icon on right instead of left (ignore arrow)
