A State Improvement in the free game C-evo is a building that can exist in only one city per nation. (The later games in the Civ series have similar classes of building under other names.) You can replace it with one built in another city.
Some of them apply only in the city they are in. Others apply to all your cities.
In Version 1.1.1 they are:
- Palace - given to your capital but can be rebuilt elsewhere; cost 75 (no maintenance cost); acts as Courthouse and Town Hall (allowing trade with no limit) there; its position determines Corruption factors under some governments
- Great Wall - needs Engineering; cost 300 + 4 per turn; acts as a City Wall for all of your cities on the same continent
- Colosseum - needs Construction; cost 150 + 4 per turn; gives each citizen one extra morale point
- Observatory - needs Astronomy; cost 225 + 4 per turn; doubles research output
- Military Academy - needs Tactics; cost 225 + 4 per turn; creates ground units already elite; speeds ground unit recovery; obsoletes and sells Barracks
- Command Bunker - needs Steel; cost 150 + 2 per turn; doubles defenders' strength
- Algae Plant - needs Synthetic Food; cost 90 + 2 per turn; adds 12 Food "for this city"
- Stock Exchange - needs The Corporation; cost 240 + 4 per turn; doubles effect of all Banks
- Space Port - needs Space Flight; cost 300 (no maintenance cost); doubles all special resources; allows you to build a space ship (which you lose if the city is captured)