Your city has become both large and respectable. The power you've collected is more than any one person could maintain or effect on their own. Your community is probably a large part of your power structure and might even influence your decision making process. In order to progress into Rank 6, you must build a Firehouse to found your Service Sector. Cities that grow too large without a fire-fighting system have burned down and had to rebuild many times in the past. Your Service Sector will prevent you from that wasted effort of rebuilding and allow you to continue to grow from city to empire.
In order to build any of the buildings in the Service Sector, you'll need to research Science. The Firehouse requires Engineering. This building reduces upkeep for all Main buildings in the settlement, provides a universal Health bonus, crafts Carts and processes Boards. It also requires Oil and Bricks to construct. Hospitals require Biology research, Glass and Tinctures. They provide a settlement-wide Health bonus and process Tinctures. Aqueducts can be built anywhere and also provide a settlement-wide Health bonus as well as increasing all effective housing space for NPCs.
Constructing both your Siege HQ and your Siege Workshop require knowledge of Science and Engineering. The Siege HQ requires Beer and Magnets to build. It provides a few free Engineers in the garrisons with every building level, trains Siege units, increases local Education and processes Lumber into Boards. The Exclusive Siege Workshop takes Boards and Ingots, increases Settlement Education, recruits free Catapults and decreases training costs inside the Siege Works. Sawmills are most efficient here but can be built anywhere and have no pre-requisite research.
The Merchant's Quarter is about turning a lot of money into even more money. These buildings are somewhat expensive and their effects profitable in that subtle indirect way. The main building is the Counting House, Requiring Silver Drachma and Wine and Mathematics research to build and improve. It increases your revenue for transportation and settlement taxes. It also processes ores into coinage like a Mint. The Exclusive Exotic Imports requires Linguistics knowledge along with Pearls and Gold Florins. Each Exotic Imports improves Shipping revenue and settlement Prosperity.
To build your Entertainment District and further levels of the central Opera House, you'll need to research Aesthetics. The Opera House also requires Pearls and Linens to build and upgrade. Each level of it sells more concessions to Players and Peers and decreases training cost for Peer troops throughout the settlement. The Exclusive Race Tracks improves Cavalry training speed and causes a few Lancers to join up free of charge.
The Park district is a fairly fancy way to dedicate more of your land to naturalism. In order to build and improve it you'll first need to study Science, then Botany. Every level of the Naturalist Center offers a valuable bonus to all Harvesting buildings, decreases Leather requirements for all settlement constructions and processes Fledglings into Pet Birds. The Exclusive Ranger Stations increases experience rewards for the entire settlement and storage within the Park district.