In the Civilization series, an obsolete unit, city improvement, resource, and/or wonder is one whose function has been surpassed or replaced by technological development. Generally, obsolete units can no longer be produced, while improvements, resources, and wonders cease to provide their effects. Obsolescence applies as soon as the civilization obtains the necessary technology; in several games, Wonders become obsolete regardless of which civ first acquires the tech.
Most games allow obsolete units still in the field to be upgraded to newer classes for a variable gold cost.
Sid Meier's Civilization and Civilization II[]
- When a unit becomes obsolete, it is removed from the city production list. Units already in production are not removed until the build order changes.
- In Civilization II, obsolete units are automatically upgraded to their replacement in city production. Leonardo's Workshop also provides its host civilization free upgrades to every unit in the field for as long as it is active; such units will lose veterancy where applicable.
- Barracks are the only improvement that can become obsolete: Gunpowder and Combustion remove the civilization's existing Barracks from all cities. Barracks under construction remain in production, and replacement Barracks can be built; these have increased maintenance costs.
- In Civilization II, Barracks are rendered obsolete by Gunpowder and Mobile Warfare. Unlike in Sid Meier's Civilization, they are sold off rather than deleted and the treasury is reimbursed.
- Wonders are rendered obsolete when any civilization discovers the appropriate advance. The wonder's bonus stops, but the wonder itself remains in its host city and continues to contribute to the civ's game score. Unbuilt obsolete wonders remain available for construction, denoted by an asterisk (*).
Civilization III[]
Unit obsolescence behaves similarly to Civilization II, with two important changes:
- Production automatically upgrades to the newest unit based on the city's available resources (e.g., a civilization that has researched Replaceable Parts but does not have access to Rubber will produce Riflemen instead of Infantry).
- If a city loses access to a required resource during production, the unit will be completed, but the build order will fall back to its closest available precursor in the next production prompt.
- Existing units can be manually upgraded when stationed in a city with the appropriate improvement for their domain (Barracks for land, Harbor for sea, and Airport for air) for a gold cost based on the difference in production cost. Leonardo's Workshop halves this cost, and does not expire.
- Additionally, any unit that is flagged to start a Golden Age (i.e., unique units) will remain available to build, even if superseded by newer units, unless/until the civ has celebrated a Golden Age.
Wonders expire only when their host civilization discovers the appropriate advance. Obsolete wonders are removed from the production menu, and incomplete wonders will prompt a change to the build order.
Small Wonders and city improvements do not expire in the base game, but can be modded to do so.
Civilization IV[]
Units[]
Unit obsolescence behaves similarly to Civ3: an obsolete unit will automatically upgrade in the city's production queue without loss of hammers, provided it has access to all necessary resources. Existing units can be upgraded anywhere within the civ's territory for a gold cost proportionate to the difference in production cost.
Resources[]
Obsolete resources continue to provide their tile yield, but the civ no longer accrues its associated trade resource, and all export deals are cancelled. The civ can still import the resource, however. Resources that can become obsolete include:
- Furs (with Plastics)
- Ivory (with Industrialism)
- Whale (with Combustion)
Buildings and wonders[]
Obsolete wonders and buildings are immediately removed from the build order, and all invested hammers are lost. Existing buildings are not removed, and continue to provide Culture output and prerequisites for other production items (e.g., obsolete Walls still enable construction of Castles, and Monasteries will continue to allow recruitment of Missionaries). All other output modifiers, such as science and experience points, are nullified.
Buildings that can become obsolete include:
- Castle, Citadel (with Economics)
- Monastery (with Scientific Method)
- Monument, Obelisk, Stele, Totem Pole (with Astronomy in Beyond the Sword, or Calendar in vanilla)
- Stable, Ger (with Advanced Flight)
- Walls, Dun (with Rifling)
Wonders that can become obsolete include:
- Angkor Wat (with Computers)
- Apostolic Palace (with Mass Media)
- Chichen Itza (with Rifling)
- Colossus (with Astronomy)
- Great Library (with Scientific Method)
- Great Lighthouse (with Corporation)
- Hagia Sophia (with Steam Power)
- Kremlin (with Fiber Optics)
- Parthenon (with Chemistry)
- Spiral Minaret (with Computers)
- Stonehenge (with Astronomy in Beyond the Sword, or Calendar in vanilla)
- Temple of Artemis (with Scientific Method)
- University of Sankore (with Computers)
Civilization V[]
Unit obsolescence behaves similarly to Civilization IV. Obsolete units in production will automatically upgrade without losing any invested Production, provided this will not create a deficit in the civilization's strategic resources. Likewise, existing units can only be upgraded if surplus resources are available (where required).
Buildings and wonders do not become obsolete, with the exception of the Great Wall (with Dynamite).
Civilization VI[]
Unit obsolescence behaves similarly to Civilization V. Obsolete units in production will automatically upgrade to their direct replacement without losing any invested Production, provided this will not create a deficit in the civilization's strategic resources. For example, the Warrior becomes obsolete with Iron Working, since this technology enables the Swordsman, the next melee unit.
Policy cards can also become obsolete as the player develops more advanced civics, replaced by new cards with more era-appropriate effects.
Buildings, districts, and wonders do not become obsolete, although Ancient Walls, Medieval Walls, and Renaissance Walls can no longer be built after Steel is researched and cease to provide defensive bonuses.
Sid Meier's Alpha Centauri[]
Base facilities and Secret Projects are not subject to obsolescence. Depending on the player's game settings, units will automatically be flagged obsolete as new components become available; they will no longer appear in the base production menu, but remain accessible in the design workshop. The player can flag and unflag obsolescence at will.
Units can also be retired, deleting the design from the workshop. This is a manual order by the player. Any units currently in the field will be instantly disbanded.