Department of Defense
Ministers in the Department of Defense don't always have absolute control over the Garrisons and standing Armies. These powers are controlled by the Civic Systems and individual laws between settlements. On the other hand, simply being permitted to manage the defense districts gives them control over most the buildings that provide Security, Security Rating and the Training of Troops for both civic and private use.
R1 Palisade Line
This is your first opportunity to add a seat to the Department of Defense. The Palisade Line is nothing more complicated than a large ring of sharpened Lumber. While simplistic, this kind of defense is useful against other early-ranked settlements with limited ability to attack each other. The Palisade handles training for all Peasant Units, leaving the Heyward as the first minister to control the production of troops. Be careful, it is definitely possible to train more troops than your settlement can support in upkeep. The buildings of the Palisade also serve to increase local Security and slow the Attributes of various attacking unit types. The district action Basic Patrol allows players to intercept attacks on any default district for a while. Building a Palisade will provide a small Overwatch rating for external districts.
R2 Main Gate
The Main Gate is your second optional Defense district. The district is centered around the Barbican, a huge gate / gatehouse that spans all of the wall and tower types, controlling the main access to your settlement. You can't build the Barbican without Masonry, so start researching Mathematics early. Every level of the Barbican can process more Bricks and reduces attribute power of Infantry and Cavalry units, increases settlement Security and decreases settlement Prosperity. Security and Prsoperity are directly opposed to each other so most Security buildings will lower Prosperity and visa versa. The exclusive Guardpost handles settlement Security, creates Prison Cells and can train all Infantry units. This means the Bailiff can can also recruit units, and in greater mass than the Heyward as more than one Guardpost can be built in the Main Gate if nowhere else. Weaponsmiths are most efficient here, so the Heyward may also find themselves responsible for producing much of the settlement's Melee Weapons. This district allows players to increase it's own Defense Rating (up to max) for a whole upkeep cycle. The Main Gate will also provide some Overwatch rating for internal and defensive districts.
R4 Military Port
Studying Mathematics, then Carpentry and Masonry will prepare you to construct the buildings of the Military Port. The main Harbor can assemble Boats and store 3 of them per building level. The building also functions as a sawmill, producing Boards from Lumber. This is handy, all the Harbor requires for construction and upgrades is Boards. The Floating Docks can be built with Lumber and provide storage for both Boats and Ships. They are a powerful local Prosperity source and increase the training speed for Boats in the Military Port. Finally, the linked Light House increases Ship luck while auto-trading and raiding and is also a powerful lcoal Prosperity source. The Admiral will be able to build Boats for their settlement but can only store Ships without being able to craft/recruit them. They may find themselves renting out much of Boat and Ship storage space. Players who visit a Military Port will be able to investigate the Economic status of nearby settlements. During Blockades, the Military Port can contribute some Overwatch to the blockade combat events.
R4 Field of Mars
The Field of Mars is primarily a training district. It's main building the Piazza Navona provides a rare Prosperity bonus to the settlement and increases housing capacity for all NPCs in all districts. It also processes Sandstone into Bricks. The exclusive Praetorian Assembly makes Commander taxes more efficient and supports two free Legionaire units to defend the Field of Mars. The most powerful building connected to the Field of Mars is actually it's linked building the Training Grounds, which are capable of training all available Unit types. Building the Piazza Navona requires a knowledge of Masonry and the Praetorian Assembly needs you to have spent a little time researching Tactics.
R5 Inner Walls
The Inner Walls are the second settlement-wrapping defense district available to your settlement. The Inner Walls are part of the several lines of defense against invasion of your internal districts. Building your Keep and the Walls that spread from it require a knowledge of Masonry, but the Moat and Guardhouse have no research prerequisites. Every level of Keep is home to 10 more free Guards to defend the district, provides Prison Cells and increases settlement Security. The exclusive Moat penalizes the Might and Celerity of attacking Peer units and increases settlement security. The linked Guardhouse provides a powerful local security bonus. This combination makes the Castellan the first member of the Department of Defense with no inherent recruitment/training abilities. Players who visit an Inner Walls can improve the district Defense Rating and Security for a day by Walking the Wall. The Inner Walls provide a high Overwatch rating for internal and defensive districts.
R7 Ranged Towers
The Ranged Towers require Masonry to begin but by the time your settlement is Rank 7, you've probably researched up to level 1 Masonry. This will allow you to build a Defensive Tower. Another powerful recruitment building, the Defensive Tower handles can train any available unit. It also is home to a few 'free' Archer units per building level that defend any attacks on the Ranged Tower district. The exclusive Ballista Tower requires Engineering research and is home to a couple free Ballista units which will pitch in should anyone attack the Ranged Towers. As a part of the ringing settlement defense, these towers will be hit before internal districts, giving these units plenty of opportunity to fight back against invaders. The Fletcher building is most efficient in Ranged Tower lots, so the Magister Inflectum may end up handling Ranged Weapon production as well as the troops they can train in the Defensive Tower. The sparse raised vantages of the Ranged Towers provide a modest Overwatch rating for the entire settlement.
R8 Militia Compound
A brief study of Tactics is all the preliminary Research you need for the Militia Compound. this centralized training district is great for training Peasant and Infantry units. The Militia HQ trains all types of available Infantry units and can reduce the Canvas cost for building upkeep throughout the settlement. The Mustering Hall reduces the cost to train Peasant Units settlement-wide and increases Peasant training speed within the Militia Compound. This makes the Magister Militum a powerful training Minister and capable of influencing the functioning costs of their fellow ministers as well. The linked building is the Armorer, so the Magister Militum may also find themselves in charge of producing the Melee Weapons for their settlement. The Militia Compound produces an Overwatch rating for combat in the internal districts.
R9 Outer Walls
The Curtain Wall lies outside the Inner Walls but behind the Palisade. Like most walls, this sturdy structure requires a knowledge of Masonry to build, maintain and potentially improve. Each level of Curtain Wall increases Security and decreases Prosperity for the entire settlement. It also processes Bricks. The exclusive Postern Gate detracts from Outer Wall security but increases all defending Infantry unit's Celerity scores per level of each gate. The linked Machiolations improve their home-districts' max Defense rating by a lot per building level. They can be built in any district but are most efficient in the Outer Walls. These fairly static buildings leave the Captain of the Guard with less influence than most members of the Department of Defense. It also leaves them with more lots to experiment with, rather than being stuck to expected behaviors. The Outer Walls provide Overwatch to external and defensive districts only.
This is the Deparment of Defense's first access to Ship construction. The Naval HQ assembles and recruits crew for new Ships but cannot itself store the ships it makes. It also increases Ship Luck and settlement Respect. Everyone is impressed by a huge ship. Of course, building ships that don't sink and come back takes a lot of knowledge. Settlements must research Science, then Physics then Navigation to start their Naval HQ. The Exclusive Dry Docks also increase Ship Luck and are each an additional Ship construction slot per level. These require research into Science, then Engineering. Finally, the linked Boathouse has no research prerequisites and can store one Boat and Ship per level. These are incredibly useful linked buildings. Together, this building pack ensures the Secretary of the Navy a powerful seat on the Department of Defense. They will be the primary source of Ships in the Department and in fact the entire settlement.
R10 Esquiline Hill
This is by far the least military member of the Department of Defense. The Esquiline Hill may have originally been dedicated to training cavalry troops, but it is now mainly a party district. The Domus Aurea is a notoriously luxurious palace that provides a bonus to all Happiness buildings in the settlement. Wineries can technically be built anywhere in the settlement, but work best in the Esquiline Hill. The Exclusive Wine Cellars enhance the production of all Wineries in the Esquiline HIll and increase the rate Peers move in in all districts. Even the district action is Attend Grand Ball, raising local Respect with each player attendee. This leaves the Magister Equitum at a loss in Defense matters but probably the best socially connected of their comrades. Building the Domus Aurea requires Masonry research but we assume you have that by now.
R11 Defensive Tunnels
The Defensive Tunnels are built completely underground. They are not visible on the settlement map and can only be reached by residents who know about them. Eventually they will be connected to the other underground districts like the Sewers, Catacombs, and half the Steamworks districts. Building both the main Armory and the exclusive Bunkers require Engineering research. The Armory is a great hidden place to keep all your weapons and weapon-crafting materials. It has tons of natural Storage and technically increases the Prosperity of the Defense Tunnels by being available to distribute. The Armory also reduces all Equipment Upkeep costs in the settlement. The Bunker also has a lot of storage and increases local Security. Each level of Bunker is protected by several Troops that will help defend the tunnels. Trenches are most efficient in the Defensive Tunnels but can technically be built anywhere. The Chief Inspector is more likely to manage items than troops with this building arrangement.
R12 Artillery Towers
Artillery Towers is the highest ranking district that provides Overwatch, but they do so for the entire settlement. They also require a ridiculous amount of Research to unlock. Every single building in the Artillery tower is behind three layers of sciences. The Ordnance Storage building at the center of the Artillery Towers relies on the Explosives Science which stems from Chemistry which stems from Science. It decreases Training costs and Health in the district and processes like a Foundry. The exclusive Cannon Towers are defended by free Gunners per level, increase settlement Security and decrease local Health. They also rely on Ballistics research which stems from Trig which stems from Mathematics. Finally, the Gun Tower can be built in any district, but only if the settlement has studied Optics which stems from Physics which stems from Science. Each Gun tower increases settlement Security but decreases it's district's Health. The Master Artillerist may not have innate recruiting ability but theirs is certainly a position of educated strength.
R13 Star Fortifications
Star Fortifications are some of the most impressive defensive constructions every produced. This rather beautiful fortifying method has worked for many city states and castles in the past. The district is managed from it's main building the Command Center. This is the final universal training building. It also provides several Prison Cells and increases local Security with every upgrade. Building and upgrading your Command Center requires the research of Tactics, which by Rank 13 is probably already taken care of. You probably also have the Engineering and Masonry researches required for the exclusive Bastion and the linked Detached Forts. Bastions are each defended by several free Spearmen per building level and increase settlement security. Detached Forts can be built anywhere and increase the Toughness of defending units. The minister of the Star Fortifications is the Chief of Staff and will be the highest ranking Minister in the Department of Defense.