Whenever I garrison a unit in a city, they seem to disappear. I had some longswordmen and catapults garrisoned in my cities for a very long time. They did not appear or have their symbol above them. I could click on the cities and nothing would happen sometimes. Is this a bug, or am I just missing something?
To the left and above the name of the city should be a circular logo with a silver border and an icon depicting a specific type of unit. if you click on that, the unit will appear and be selected. In the case of air units, you need to click on the black, numbered bar below the name of the city to get a list of all available air units.
Near the city's name, there is a small indicator( zoom in a bit to see it clearer) that says what units are garissoned (this works especially well with several airplanes on that city, that's where i noticed it) and you can select through them.