training units can be done from 3 locations.

Units Training is referenced with a black Spartan helmet icon.

Units Training requires at least 1 Barracks Urban building. Cavalry units also require Stables, Siege unit also require Forge.

while in WorldView,

tap YOUR city, at the bottom/center of the TileMenu, will show the black Spartan helmet will train units in this city.

tap the bottom left city icon, pick the city from the list on the left, then scroll down to Services Overview

while in CityView,

tap any barracks.