City garrisons

I'm new to Brave New World and can't seem to find the answer to this, either in the Civlopedia or on here. In Civ 5 there was an order button to garrison a military unit in a city. In Brave New World I can't seem to find it. It is not one of the orders for a unit currently in a city. Am I missing something, i.e. some tech or building as a prerequisite for the ability to garrison? Your help is appreciated.

Regards