Cities are your main source of income and unit production in Gladius and destroying them is also the default victory condtion.
Founding cities
In order to found cities, most factions first need to research and produce units with the ability to do so (Techpriest Enginseers, Meks, Canoptek Spyder), although everyone starts with one. Necrons can only build their cities on tiles with Necron Tombs. Space Marines drop their city from orbit, but can only do it once per game. Cities can only be founded at least 4 tiles away from already existing cities and every city besides your first reduces the 
in every city by -6.
Improving cities
A new city starts out with its main building, a building construction building and only a single tile. Each tile can hold up to 3 buildings, but to be able to build on them, they must first be acquired by the city. A city cannot acquire tiles outside of its aquisition range, which starts out at 2, but can be increased to 4 via research (up to 5 for Space Marines). Every tile acquired by a city costs an upkeep of 1 
per turn, but also gives units stationed on it a powerful 
Damage Reduction and 
Healing Rate bonus, reduces movement penalties and has zone of control, slowing down all enemy units next to it. The production output of buildings is influenced by the resource yield of the tile it's built on, which depends on the tile's terrain. Note that most buildings produce resources, even unit production buildings, which gives you a reason to keep them active even when you are not using them to produce units. A special feature acquired by a city will give its percentage bonus to the whole city's production.
Every production building in a city has its own build queue and constructing additional copies of production buildings speeds up their 
rate, but running out of resources will lead to an up to -50% malus.
Population
Population grows as long as the city has free housing. Housing is obtained mainly through the main building and housing buildings (Hab Block, Dormitories, Grotz Hutz, Shelters). Population 
Growth rate is reduced when there is less than 5 free housing. Every turn the city generates +8 Growth and once that number reaches 32, a new population unit is produced. For every free housing less than 5, population growth is reduced by -1. Orks have increased population growth. Acquired Fermentation Pools increase the city's population growth by 20%.
Having more active buildings than current population in the city will reduce the resource output of all buildings by a percentage. To prevent this, you're able to choose which buildings are active and which aren't by clicking on the minus and plus signs in the city menu. Inactive buildings will not produce resources, but they won't require population and also won't cost any upkeep.
Every unit of population costs upkeep, normally in the form of 1 
and 1 . Instead of , Space Marines take 1 
as upkeep and Necrons 1 
.
Loyalty
It is very important to pay attention to your city's . It can range from -50 to +50 and each point influences the city's entire recource production output and construction speed by 2%. Having a city at +50 loyalty means its entire production is doubled, so often it can be more profitable to build a loyalty building than another resource production building. Loyalty is increased by loyalty buildings (Sanctum Imperialis, Company Chapel, Fighting Pitz, Baroque Shrine), but also by Space Marine Chaplains, Recaf Leaf special resources and by the Siren Caster Artefacts. Loyalty is reduced by -6 for every other city you own and by -1 for every population unit in the city.