The artificial intelligence (AI) in Civilization V is designed to operate on four levels: the tactical AI controls individual units; the operational AI oversees the entire war front; the strategic AI manages the entire empire; and the grand strategic AI sets long-term goals and determines how to win the game. The four levels of AI complement each other to allow for complex and flexible AI behaviors. Each of the AI-controlled leaders has a unique personality, determined by a combination of 'flavors' on a ten-point scale; however, the values may differ slightly in each game. There are 26 flavors, grouped into categories including growth, expansion, wide strategy, military preferences, recon, naval recon, naval growth, and development preferences. As well, certain AI leaders may not get along with other AI leaders, as is the case with Alexander the Great of Greece and Darius I of Persia, as those two represent cultures that were historically enemies.
As in previous versions, cities remain the central pillar of Civilization gameplay. A city can be founded on a desired location by a settler unit, and the city will grow in population, produce units and buildings, and generate research and wealth. The city will also develop culturally and expand its borders one tile at a time, which is critical in claiming territory and resources. The expansion process is automated and directed towards the city's needs, but it can be accelerated with gold. Cities can use up to three tiles outwards, instead of two.
City warfare has been revamped. Whereas cities in previous versions of Civ relied entirely on garrisoned units for defense, cities in Civ V now defend themselves, and can attack invading units with a ranged attack expanding two tiles outward. Cities have hit points that, if taken down to zero, will signal the city's defeat to invading forces; surviving an attack allows a city to recover a fraction of its hit points automatically each turn. In addition, any melee unit loses hit points upon attacking a city. Hit points can be increased by garrisoning a unit in the city or building defensive structures. Captured cities can be annexed, razed, or transformed into puppet states, each option having distinct advantages and disadvantages; for example, puppet states will provide resources and have lower unhappiness, but cannot be directly controlled.
In this iteration of the series, tactical gameplay is encouraged over numbers, with the introduction of new gameplay mechanisms. Most significantly, the square grid of the world map has been removed in favor of a hexagonal grid, a feature inspired by the 1994 game Panzer General, according to lead designer Jon Shafer. In addition, each hexagonal tile, including city tiles, can accommodate only one military unit at a time, forcing armies to spread out over large areas instead of piling onto a single tile. This has the effect of moving most large battles outside of the cities, and forces increased realism on sieges, which are now most effective when surrounding the city tile. Also, increased movement points, simpler transportation over water, ranged attacks, and swapping of adjacent units allows for more versatile maneuvering of units. There is also a balance between ranged and melee units. Early ranged units can attack melee units without retribution, but melee units will outright destroy ranged units.
Units take longer to produce than in previous games from the series, making them more valuable. As they defeat enemy units, units may be either promoted for bonuses or forgo their promotion to be completely healed. Another departure from previous games is that units are no longer always destroyed if defeated in combat.
Special "Great Person" units are still present in the game, providing special bonuses to the civilization that births them, with each named after a historic figure such as Albert Einstein or Leonardo da Vinci. Great people come in several varieties, and can be consumed to produce one of three effects: start a golden age, build a unique terrain improvement, or perform a unique special ability. For example, a Great General can create a 'Citadel' (an improved fort with the ability to inflict damage on nearby enemy units), or increase the combat strength of nearby friendly units (this is the only ability that does not require the consumption of the unit). Capturing a Great Person destroys it.
Compared to previous titles in the series, technology trading has been removed in favor of joint technological ventures. Two civilizations at peace can form a research agreement, which for an initial investment of gold provides both a certain amount of science so long as they remain at peace. Prior to the 1.0.1.332 PC version of the game, research agreements provided both parties with a random unknown technology after a set number of turns of uninterrupted peaceful relations. It is possible for a civilization to sign a research agreement for the sole purpose of getting an enemy to spend money which could be used for other purposes; AI civilizations are programmed to sometimes use this tactic before declaring war. After the player discovers a new technology, a quote related to the technology is read by British actor W. Morgan Sheppard.
City-states, a new feature to the series, are minor civilizations that can be interacted with. Unlike major powers, however, while they may expand in territory they never establish new cities. In addition to outright conquest, major civilizations have the option to befriend city-states, via bribery or services, for bonuses such as resources and units; these bonuses increase as players advance to new eras. There are three types of city-states, each with different personalities and bonuses: maritime, cultured, and militaristic. A city-state has the potential to play a prominent role in diplomacy among larger civilizations, as well as make specific requests and grantrewards.
In a change to the culture system, in Civilization V players have the ability to "purchase" social policies at the expense of earned culture. These social policies are made up of ten separate trees (some trees are mutually exclusive), and filling out five of the ten trees is a requirement to win a cultural victory. These policies replace the "Civics" government system of Civilization IV; the main difference is that the player had to switch out of old civics to adopt a new one, while social policies are cumulative bonuses. According to Jon Shafer, "With the policies system, we wanted to keep the feel of mixing and matching to construct one's government that was part of Civ IV, but we also wanted to instill a sense of forward momentum. Rather than having to switch out of one policy to adopt another, the player builds upon the policies already unlocked. The thought process we want to promote is 'What cool new effect do I want?' rather than the feeling of needing to perform detailed analysis to determine if switching is a good idea."
As in previous games, there are multiple ways to achieve victory in addition to military conquest. The player may focus on scientific research and become the first to assemble and launch a spaceship, winning a Space Race victory. Diplomatic victory requires support from other civilizations and city-states in the United Nations. In the revamped culture system of Civilization V that consists of social policy "trees", the cultural victory involves filling out five of the ten "trees" and completing the Utopia project (reminiscent of the Ascent to Transcendence secret project in Sid Meier's Alpha Centauri). World domination is of course an option, but the victory condition has been simplified compared to previous games in the series. Rather than completely destroying the other civilizations, the last player who controls their original capital wins by conquest. The player can also win by having the highest score at the year 2050 AD. (Which is the Time victory, which can be disabled)
There are 18 civilizations available in the standard retail version of Civilization V. The player chooses a civilization and assumes the role of its leader, based on prominent historical figures. Each leader of a civilization has a unique unit, another unique unit, building, or improvement and a special ability. The player is able to interact with the leaders of other civilizations via the diplomacy screen, which features — for the first time in the series — fully animated leaders that speak their native languages. For instance, Augustus Caesar speaks in his native Latin and Montezuma speaks in his native Nahuatl. According to Émile Khordoc, who voiced Augustus Caesar, the voices for the leaders were recorded in early 2009, approximately one and a half years before the release of the game.
