You can combine units into a group by pressing SHIFT and dragging the mouse over them. You assign the group to a certain number by pressing CTRL and a number key (1 to 9). If you want to switch back to this group later, you simply have to press the group number. You can always deselect a unit from a group by pressing SHIFT and clicking at the unit. To select a group, just press the corresponding number key. If you press ALT plus the number key, the view will even be centered on that group. You may think that nine groups are more than you need, but in fact it is not too much.
Loomer has mentioned on Area S that he separates the units by their speed and makes four groups, one for the fast Cavalry, one for Infantry and Archer units, one for slow infantry and one for Siege Engines. I go even further and separate them by their function:
Unless you farmers are close to a unique building to which you can switch with a hotkey (e. g. CTRL+K for Siege Workshop or H for the Town Center), you should assign one of your granaries to group 9.
To assign a hotkey to a granary (or any other building that does not have a system hotkey), highlight it and then type Ctrl+1 through Ctrl+9. Then use Alt+<number> to jump to the granary. (Raymond)
In the Stone Age, give every new unit its own number so you can scout your neighborhood very efficiently. Concentrate your troops as soon as you have more of them or find the enemy :)
Also use a hotkey for your war ships (e. g. 8) and your transports (e. g. 4). The numbers are of course only suggestions; use the number that you like. Profit from the high number of groups available and do not make only one or two of them! You will have an advantage in combat then when speed matters.
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