Kambrea

We drop from warp near the edge of the system and from the moment the lights turn back on, I can tell this is a bad idea. To say the system is crowded would be an understatement, even when you consider just how vast space is.
Kambrea is the trade capital of The Dhitol Trading Guild, an ecumenopolis with over a thousand ships coming and going at any point in time. That's a lot of ground to cover.
I turn to Fraeia who stands behind me and ask, "How do you expect to sift through every ship? Even if we pinpoint where they make the antimatter, how are we going to find a target?"
They reply as they sort through a list of transponders, signals that display the name of every vessel in the system, "We know now that antimatter weapons are still in the developmental stages of production. We're looking for a military vessel, which narrows our list down significantly. Scan the planet for the highest concentration of energy output."
I give them their answer in moments, "Northern hemisphere, Plate 13, sublevel 8."

There are two ecumenipoli currently known in the galaxy. The first is Safeharbor, humanity's adopted homeworld. The second is Kambrea, the capital of The Dhitol Trading Guild. The two couldn't be more opposite. I spent a good chunk of my life living on that rusted planet of metal and faulty machinery. It's strange to see what Safeharbor might have looked like in its prime.
Nestled near the red dwarf Xhoksa, Kambrea is the second of three planets in the system. There used to be five. The others were strip-mined, torn apart to feed the beast that is Kambria. It's twice the size of Safeharbor, housing more than fifty billion citizens, most of them being Dhitol.
The entire surface is metal. Any flora you find was put there after the planet's conversion was completed. The fauna consists primarily of lost pets, exotic creatures that found a way to grow their population in secluded corners of lower levels. The lower you go, the more pollution, poverty, and danger there is.

Origin

Kambrea was originally the territory of The Dhitol Confederacy, which collapsed less than a few centuries ago. It's rare for a trading organization to represent an entire people. In the waning days of the confederacy, the trading guild amassed such power and influence, that it managed to annex the majority of the confederacy without a fight.
This is when construction began on the ecumenopolis, an endeavor that took extreme feats of engineering considering the whole planet was both being terraformed and inhabited at the same time. Even with their priceless merchant fleets along with all the resources and labor they could need, the project took two hundred and seventy-three years.
Those involved lived and died working to make this dream a reality. They spent their lives focused on making a better, brighter future, not for themselves, but their children and their children's children. Granted their descendants decided to poke a sleeping giant by provoking the eden, but… still.

Cutting Costs

The excuse goes like this: With so much ground to cover, there's bound to be things overlooked. Accidents happen. There's always an accident here, a tragedy there. They're big enough to matter but too small to make a difference. Then you have the lower levels. The lower levels are often where those who slip through the cracks go to die quietly, out of sight and out of mind.

It's a horrible place to live. A dystopian cesspool dominated by small corporations and criminal syndicates dedicated to keeping the poor as poor as possible. Ironically, these levels are where the support for the plates above are located. It takes one bad day and one angry citizen to change the game, here. Eventually, someone will look up to rock formations and the supports holding the plates above then wonder, "what if I break it?"
Naturally, this takes a delicate approach, and with that, we're back to money. Crime and terrorism are rampant in the lowest levels, gangs fighting to control territory to get a little bonus from the trading Guild to ensure they don't detonate a bomb on the supports. Crime pays here, and there's no law willing to delve into the depths to change that fact.

The City

A planet-wide city needs constant maintenance and upkeep, so why bother? It's simple. Money. It's always money. Overpopulation is one thing, that's what colonies are for, but this? This planet is an economic powerhouse.
An ecumenopolis is like layering the surface of the planet on itself several times over. Every level provided more space and more opportunities. Every level is divided into districts, some focused on housing and recreation, others on industry, infrastructure, and R&D.
Can you imagine an entire planet's surface worth of factories, each producing different goods? Now imagine the next level below is an entire planet's surface worth of research laps and below that an entire planet's surface worth of residential districts to house those who work on the levels above. It's costly, but the profit is just as exponential in growth.