In the top left of the Army Management screen we have the troops you are using in your current army when attacking: Below the currently selected army, we have Defending Army which is used in the defence of your city. You select troops by clicking on it then going picking them. Below the Defending Army is the Heal button which allows you to heal damaged troops with diamonds. Below the Heal button is the delete unit button which allows you to select and delete units from your army management. Here you can select to show only units of a specific type. From left to right these are: All, Fast, Heavy, Light, Artillery and Ranged. These two drop down menus allow you to further filter troops by All units, Unattached units, Attached units and by era. However you will need Unattached units to be shown by selecting this in settings. If you scroll along to the end of Army Management you will come across this symbol. If you set up the Army Management to a specific era and hover over it. You can see all the troops you have in that era.