Defensive:
Bastioned Fortification Also known as the 'Italian Trace' or 'Vauban Fortification'. Salable with Gunpowder, since even primitive gunpowder 'Bombards' make all previous city walls and towers obsolete. Very expensive to maintain, but provide lots of positions for cannon, so increases both the city 'Hit Points' and ranged factors enormously (100% or more).
The Castle becomes an Improvement. Like roads, can be built in any Tile without disturbing other Improvements, makes it impossible to Raze anything in the Tile until the Castle is reduced to a Ruin. Castle, though, has defenses like a small city. All Castles automatically become Ruins when Gunpowder is discovered, but the Ruins become generators of Gold and Tourism once Railroads are discovered.
Ring Fort - the last iteration of direct city defense: 19th century-type concrete and steel, cannon-armed fortifications. Available with Gating Guns or Railroad, hideously expensive to build and to maintain, so only a few cities got them historically (Paris, Liege, Verdun, Metz, etc)
Save Armories and Arsenals to provide Promotions for the troops...
Growth:
Sewer System - available from Construction, because Babylon had one in 600 BCE! Required for a City to grow past a certain size, requires continuous maintenance, but can be Upgraded/Supplanted by Water Works in the early 19th century (beginning of 'Industrial Era', which allows another increase in size, perhaps to twice what the original Sewer System allowed.
Mass Transit - available with Electricity, combined with Water Works removes all restrictions on city size, increases Happiness and Gold in city.
Supermarket - available from 1950's, increases Food from all sources by 25%, requires no maintenance since they are (supposedly) profit-making enterprises.
Production:
Power Plant - available with Steam Power, requires Coal OR Oil, required for Mass Transit and increases Production from Factories by 50%. Can be replaced later by Hydroelectric Station, Nuclear Powerplant, or Solar Powerplant.
Gold/Trade:
Textile Mill - the 'Enlightenment Era' Mod has this, but I suggest its effects should be a little different: For each Sheep, Cotton or Silk resource, the city with a Textile Mill produces one Cheap Cloth 'Luxury' Resource AND Gold in the city. Once Factories can be built, the Textile Mill can be converted to a Textile Factory with all the benefits of the Factory (increased Gold from Power Plants, for example). You can have both a Textile Factory and a regular Factory in the same city.
Container Terminal - available in the mid-1950s, so about with Jet Aircraft or just after. Increases Gold from Trade Routes into/out of the city by 10% for each type of transportation: Railroad, for example, would increase All Trade Routes by 10%, if also have a Harbor, +20%, if also an Airport = +30%. Containerization dramatically increased the speed with which freight could be shifted from one carrier to the other, so a major hub moving freight from ships to airport to railroad will gain from each.
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