Through friends, determination or chicanery, you've reached Rank 3 and are ready to build your Marketplace. This is a reasonably large undertaking, since you need Bricks for it. It's possible that Bricks both came in your sett-starter-pack and that you've managed to hold onto them for this long, but more likely you've been scrambling to harvest sandstone and have got a single Brickmaker somewhere preparing you for this moment. Once your Forum is complete, you have access to four other districts to choose from. This post is here to outline what that choice is for a burgeoning sovereign.
The Marketplace is the Rank 3 Default District. This, as you've probably determined, means that you can't have a Rank 3 settlement or access to the other Rank 3 districts without it. You also can't skip it and move on to Rank 4 without one. The people simply demand to have a location to meet and trade things. This is great for Prosperity, but terrible for Security. The Forum is the central building and provides the incredibly helpful function of chiseling Bricks from sandstone. As it is built and kept up with Bricks, this building will self-maintain as long as it's owner/manager has Sandstone. Both the Forum and the exclusive Bazaar have Income Building functions, selling goods to Peasants and Yeomen for currency. Linked Jewelcutters are more efficient in the Marketplace and are the only building that can produce Cut Gems from raw ones.
The Spotting Tower is an excellent addition to your Department of Education (if you have one yet). The main Cartographer's Tent increases the Might of all defending Siege units and increases your Action Points when your settlement is on the attack. Both the Cartographer's Tent and the Spotting-Towers exclusive Heliograph Towers provide a neat settlement-wide Education bonus but both buildings require difficult to craft goods to build and upkeep. Every Watch Tower will boost settlement Security but are most efficient when built in the Spotting Towers. Building this district also gives you access to the Basic Recon ability, allowing you to gather information on neighboring settlement Garrisons.
If you like Boats and Carts, the Canal District is right for you. The Main building is the Canal Infrastructure which not only installs little Canals for your boats but provides a production bonus to pretty much every type of processing building and stores boats. It also requires Slabs to build. The Cargo Dock, which only needs Leather, builds Carts and increases local Boat income by a few Thalers with every level. Fisheries are most efficient in the Canal District but can be built anywhere. These handy buildings process all your oceanic resources into their appropriate Goods. Players in this district can perform Custom Inspections on the boats that float through the Canals and go home with a few random confiscated goods.
Every building from the Plaza district is helpful and powerful in it's way. That is, if you have the Slabs for it. While somewhat costly, the Central Fountain is also a great building. On it's own, it collects the tossed Copper and Silver coins thrown in by local NPCs. It then increases how quickly Peasants move into free space and provides a settlement-wide Health bonus. Finally, because the Fountain is the center of the Plaza, players can visit, drink from it, and regain both Health and Magic points by a percent of their total capacity. Then there's the Colonnade. This is an outdoor 'storage' area made of solid marble Slabs. Heroes can beneath their roofs in the 'open air' while still being considered housed. These residents earn more Importance, while the presence of these marble monoliths increases Respect happiness in all districts. Finally, Brickmakers (all important) are most efficient here in the Plaza district. At least they only require Sandstone to build.
The Temple District is your one-stop for restoring settlement Faith. The Pantheon on it's own provides a reasonable static bonus per level to all district Faith and a small percent Faith bonus to the Temple district itself. It also can house a small number of Exemplars and those residents gain an AP capacity bonus. The exclusive Churches increase full-settlement Faith as well as processing Herbs into Tinctures. Finally the linked Chapel is most useful (if not optimally efficient) in districts suffering from low Faith, as it's powerful local bonus will help to heal that deficit.