If your settlement has achieved Rank 7, your Sovereign Character has also reached Rank 7 and has built a Mid-Town District. This required district represents the necessary center of paperwork management. Your settlement has gotten so big that it will not be able to support it's own administration without a Town Hall and supporting buildings.
Building your Town Hall requires knowledge of Civics, which is based on Science research. This building starts and is the center of your Mid-Town district and makes taxes from all Noble player residents more efficient. Civil Administrations can only be built in the Mid-Town district and help the Town Square and reduce transport losses for everyone in the settlement. Apartment buildings are most efficient in the Mid-Town but can be built anywhere.
The Ranged Towers is the optional defensive district of the Rank 7 set. Building your Defensive Tower will require some knowledge of Masonry but will provide you with free defensive Archers, train Artillery units and increase the Toughness of all friendly Ranged units. The Ballista Towers require Engineering research, recruit free defensive Ballista units and reduce the Toughness of attacking Peer units. Fletchers, your best bet for crafting Ranged Weapons, are most efficient in the Ranged Towers district.
Reward your wealthy citizens by building a Boulevard District. The central Department Store requires Glass and Brick to build but has no pre-requisite research. This is is a great chance to craft Equipment, the Dept Store produces all gear types except Spells. It also improves crafting efficiency for Accessories and sells Canvas as if it were an Art Gallery. The Exclusive Boutique crafts only the Accessory type of gear, but improves Charm for every citizen in the settlement.
With a city so big, eventually many people will have died within it. Using ancient tunneling knowledge, you can build a cool underground facility to house your dead. While the main and exclusive buildings have no pre-requisite research, the lnked Burial Chamber requires Masonry. The Inner Sanctum and Burial Chambers can both house player dead and all three buildings create benefit based on deaths in the settlement.
The research prerequisite for Sewage Pipes is Engineering, unlocked through Science. These tunnels extend beneath your entire settlement and increase the Health and Population housing capacity throughout. While they require Iron Ingots and Oil to build, it also processes like an Oil Press, providing to it's own production chain. Cesspits can only be built inside the Sewer district. These filth-containment installments can store carts and increase the rate at which your Peasant Population expands. Fountains are most efficient in the Sewer district but can be built anywhere.