Carlson Citadel started out as a small walled town long ago, guarding one of the few safely traversable routes through the Badlands to one of the Pyricorn Threads that afforded safe travel to and from the Kingdom In the Sun. The city served as a waypoint for travellers from Rence and the Corona, but did not have much in the way to make it stand out. The most that could be said about Carlson Citadel was that it was a good, steadfast town, with a small theatre, a good bar, and a handful of metalworkers who mined the badlands and attempted to refine and shape the ores found there into useable metals.
This changed once those same metalworkers realized the kind of resources that they really had access to:
The steam vents of the Ara'Tsym Badlands.
Like Amber, Ember was not a location that was naturally conducive to heavy industry. Influenced by the Queen's subconcious desires upon the redrawing of the Fire Pattern and the Restoration of the Kingdom, Ember shared many of the same physical laws as The One True City. Gunpowder as it was known in Shadow did not ignite, nor would imported coal. Fossil fuels in general were hard to import and manage given the kingdom's temperatures, and the few experiments with alcohol-based engines had proved fruitless. Steam power -was- possible, but without a reliable fuel to boil water (a precious resource in Ember, especially after the loss of Rebma) in massive quantities, any sort of 'industrial revolution' that could have formed in Amber or Ember had been cut off at the metaphorical knees.
This just meant that the engineers of Ember had to get ... creative.
There was no need to boil water to create steam near Carlson Citadel, for the steam already existed, issuing from the ground in high-pressure vents scattered among the arrayos and valleys of the Ara'Tsym Badlands. The metalworkers and industrialists of the city took advantage of this, laying long pipelines into the bandlands, capping the geysers and steam vents, forcing the hot, pressurized gasses into large, heavily-reinforced pressure vessels to allow for a controlled release of energy.
With that hurdle cleared, Carlson Citadel began to grow at an astounding rate. It continued to be a walled city, but the city's boundaries expanded, as did the walls, the engineers utilizing the high-pressure steam equipment to astounding effects, to refine metals better and to build structures higher. By the time of Queen Corrine's return to Ember, the Citadel was effectively a soaring metropolis, filled with engineers and industrialists of every stripe. This concentration of mechanical talent served the city well in the decades to follow, as the Queen began to implement several new directives, combining Trump and machinery in innovative ways, producing the foundations of Ember's cross-Shadow Aerocruiser and Zeppelin fleet.