Expansions

Expansions are buildings that need to be placed in the physical space of the game world to be built, rather than City Improvements, which are simply housed in the City Center.

Different types of expansions provide different benefits to the city they're built in, such as improved FIDSI output, Strategic and Luxury resources, or Vision over an area, and each type of expansion has certain rules on where they can be placed.

Districts
Districts are the most common type of Expansion, making up the physical form of a City. All cities start with only a single district - the City Center - and grow out from there by placing districts adjacent to the City Center or other districts. Districts can only be placed on tiles within the same region as the City Center, and cannot be placed on Ruins, Minor Faction villages, Ridges, or - except for the Cargo Docks - Sea/Lake tiles. Once a district is built, the tiles surrounding it that are not occupied by other districts and in the same region as the City Center will be added to the city's area of exploitation. Additionally, a district built over a watchtower foundation, strategic resource deposit, or luxury resource deposit will automatically act as a watchtower or resource extractor, respectively, so long as the appropriate technology is unlocked.

All districts can be leveled up by surrounding them by 4 other districts of the same level or higher, and provide additional FIDSI (and sometimes Approval) output at higher levels. While most factions can only upgrade districts to level 2, the Cultists can upgrade districts to level 3 due to their "High Seat of the Queen" trait.

The most common type of district is the Borough Streets, which become available for construction by increasing a city's population; every 2 population for most factions, every 1 population for the Necrophages due to their "Cellulose Mutation" trait. The Allayi instead use the Garth of the Allayi, which is available every 1 population. Both have Industry costs that increase for every additional Borough/Garth built.

Unique Districts
Aside from the Borough Streets and Garth of the Allayi districts, there are a number of districts that can only be built once per city, or once per Empire in the case of the Altar of Auriga. They usually have specific requirements to be built, such as being placed on a specific kind of tile, and most are unlocked from Blessings.

Legendary Buildings
Finally, there are the Legendary Buildings. Constructing one of these districts is considered a Legendary Deed, and as such, each one can only be built once in the world. There is one legendary building for Eras I through V, and as with other Legendary Deeds, are available for any empire to build as soon as the building's era has been reached by any empire in the world.

Resource Extractors
Resource extractors are buildings that need to be built over resource deposits in the same region as the City Center. Upon completing a resource extractor, it will produce 1 unit of the associated resource per turn. Placing a resource extractor on a tile will not prevent a district from being placed there.

For each resource extractor, the appropriate technology must be unlocked before it can be built.

Towers
Towers are built on foundations, which are revealed upon reaching Era II. Both types of towers provide an area of Vision starting from the tower, but Watchtowers and Defensive Towers provide other distinct benefits. Additionally, Watchtowers are unlocked through technology, while Defensive Towers are unlocked through blessings.

Placing a tower on a tile will not prevent a district from being placed there.

Watchtowers
Aside from the vision provided by Watchtowers, they also provide bonus health regeneration to friendly units on the tower and its adjacent tiles, as well as Detection within their vision range. There are three versions of the watchtower - each better than the last - which are unlocked by three different technologies; upon unlocking the technology for the next tier watchtower, any previously-constructed watchtowers will be automatically upgraded.

Defensive Towers
Defensive Towers do not provide as large a vision range as normal watchtowers, and do not provide health regeneration or detection. However, they do inflict Retaliation Damage to hostile units in their vision range. There are two tiers of defensive towers, and - as with watchtowers - any previously-built defensive towers of the first tier will automatically be upgraded to the second tier once the appropriate blessing has been unlocked.

Fungal Blooms
The Mykara are able to create Fungal Blooms due to their "Creeping Presence" Affinity. Unlike other types of Expansions, these Blooms can be built in any region that is either neutral or under the control of the Mykara, and instead of using Industry to be built consume some Food  from the main city for a number of turns. These Fungal Bloom Expansions often replace other types of expansions, such with resource extractors. Fungal Blooms don't get bonuses from any city improvements, like Plow Factory or Apprenticeship Registry.