The marketplace is the first district you must build if you want to move your Settlement beyond Rank 2. People gathered in large enough numbers will create themselves a market place, whether a town square, a church courtyard, or an entire designated field. Your Settlement has finally gained enough momentum that the people both demand and can use this large space in which to conduct their daily necessary trades.
Step right up! Plunge yourself into the teeming masses of daily trade! Everyone can tell you're packing cash. Every trader will tell you they've got the best deal. By the time you get back out of the bustle, your money is gone and a piece of gear is sitting in your hands. Is it awesome? Can you even wear it? These are the risks taken when Bargain Hunting.
The forum is your center of trade and discussion. People come here to talk, shop, and find out what's going on in their Settlement. Generating Drachma and processing Bricks, the Forum is a uniquely useful main building. Upgrading this building increases Prosperity city-wide, as well as producing yet more Drachmas and Bricks per cycle.
Let the people trade! All it takes is building a Bazaar, and the business of the people will generate Drachmas and Litra as commerce commences. Every level of Bazaar increases Settlement Prosperity, while only risking the Security of the Marketplace. Bazaars are the most direct representation of that which pops up on every street corner where free trade is allowed.
The Jewelcutter is the only building in the game that processes Raw Gems into Cut Gems. So if you want those incredibly refined decorations (and valuable sparkly trade good), you'd better build some. Gems, while a high end commodity, matter a great deal to some casters and to participation in really snazzy actions/scenarios.
Concerned only with the fair flow of cash, the Accounting ministry is a key portion of the Department of Commerce. Not everyone can keep those long columns of numbers straight. The Ministry of Accounting is full of those wondrous little counters.
Overseeing the vast comings and goings of the Marketplace, the Comptroller is a demanding position. Especially since no Settlement can see Rank 3 without one. Comptrollers beware. Everyone thinks they can take if off the top when so much coin is changing hands.
Where there is commerce, there is theft. Every building native to the Marketplace is incredibly good for commerce and an incredible temptation for the criminally inclined. Without Guardhouses, this district can quickly become nothing more than a training ground for new light-fingers.