Upgrades are a way of improving Cities. Ruler's may spend a large sum of Shmuckers directly to upgrade a city's level. Once a ruler orders an upgrade, the treasury is debited and the upgrade happens immediately and automagically via Moneymancy.TBFGK Epilogue 1 Upgrades can be done at most any time, even at night, so long as no enemy units are present.LIAB 33a
Upgrading a city's level increases the city's defensive bonus, the ammount of Shmucker's it produces, and the variety of units it can be scheduled to pop.
Sides with Dirtamancers present in the city can reduce the cost of the upgrade, and if they are linked with a Thinkamancer this will further reduce the cost and improve the Signamancy of the upgrade.LIAB Prologue 4 Any trap or improvement a Dirtamancer creates in a city also counts towards a potential upgrade, making it cheaper. If someone with lesser knowledge of Dirtamancy attempts to upgrade the city later, they risk damaging any of the Dirtamancer's custom work.
A Trimancer link can create unique upgrades to a city, such as Jed the Head.
When and whether to upgrade a city is an important strategic question for a side. It's an expensive investment with a long return on investment, and the Shmucker's may be better spent elsewhere. It is unknown how upgrading a city may affect or be affected by diminishing returns.