Units pop at the start of a Side's turn, as part of the normal production of a city. Units pop with a set of non-magical equipment such as standard armor, weaponry, and clothing. Cities may randomly pop extra units, requiring the Ruler to decide to pay a one-time extra cost or disband the unit.
Rations and Unit Upkeep
Units receive physical food rations and other objects required for their daily functioning at the start of their turn. These objects may be consumed at any point in the day, or saved.
Barbarian units may also pop randomly in non-city hexes. Non-unit wildlife pops commonly, with the type, number, and frequency of the plants and animals dependent on the Terrain Type. Magic Items, Artifacts, and Gems may pop randomly in Ruins or the sites of sacked cities.
The disappearance of objects is called unpopping. This happens automagically to corpses of Croaked units which are not Uncroaked, eaten, moved from the city or hex where they were croaked, or claimed as trophies. Unpopping of corpses happens at the start of the next turn their Side has. The disappearance of living units is called disbanding.
The units that can pop in a city depend on that city's Level.
It seems that once a unit has been selected for production, however, it will remain in production until orders are changed, even if the city loses levels to below the required level.
When Gobwin Knob fell to Level One during the Volcano Uncroaking, they could only produce Infantry and Twolls. However, the next Turn, they popped a Blue Dwagon while still Level One. So, the Level One city was able to create enough production to fulfill the remaining requirements of the dwagon and it popped despite needing a Level Three city to be produced. They could not, of course, begin producing a new dwagon at that point, though they could begin producing Twolls.
An established side had a very remote random chance of popping a caster any time they pop a warlord.
The size, forces, location, aggression, etc. (or the Ruler's heart and fate) can increase or decrease the likelihood of popping a caster. (Faq was tiny and nonaggressive - King Banhammer had no use for warlords. He had three casters - though one was potentially a Magic Kingdom hire and another may have come from a different side).