challenge of adding new content to improve upon what's already there without bloating it. Civ VI's first expansion, Rise and Fall, strikes a remarkable balance between the two, with several key features that both complement and change up the base game.
The big-picture addition and namesake of Rise and Fall is the Ages system, which coincides with each of the existing technological eras--Ancient, Classical, etc.--but is based on a global average rather than individual progress through the tech tree. As the world collectively transitions from one era to the next, each civilization accumulates a score toward the next era's "Age." Depending on your progress during the previous era, you can enter a Normal Age, fall into a Dark Age, or rise into a Golden Age. While Golden Ages obviously carry the most benefits, Dark Ages have unique bonuses of their own, and if you pull yourself out of a Dark Age and into a Golden one, it'll be even stronger. These so-called Heroic Ages are a powerful weapon later in the game if you've fallen behind and are struggling to catch up.
The Ages system works brilliantly with Civ VI's emphasis on careful planning and building a well-rounded civilization regardless of the victory condition you're working towards. A wide variety of accomplishments contribute to your score, from clearing Barbarian outposts and building Wonders to being the first civilization with a complex form of government. If you lean too heavily into one specialization, like science, you'll have trouble earning enough points in any given era to escape a Dark Age and its pitfalls. So even if you're two eras ahead of everyone else in your own tech tree, you're still susceptible to falling into a Dark Age if you fail to do anything else of note. As a result, a strong start isn't enough to carry you through Rise and Fall, even on lower difficulties--you need to work proactively and adapt your strategies at every step if you want to rule the world.
Building off the base game, monitoring each city's individual growth is paramount. In vanilla Civ VI, cities have individual happiness meters instead of civilization-wide ones, and greater depth to city development forces you pay close attention to each city and its unique contributions to your empire. Rise and Fall adds two big features that affect cities specifically: Loyalty and Governors, which work in tandem to add depth to city management without overcomplicating it.
Loyalty is a metric of each city's dedication to your leadership and is added on top of happiness to the list of city stats you need to care about. Loyalty suffers in Dark Ages and flourishes in Golden ones; if it falls too low, your city will be less productive and eventually revolt, becoming a "free city" open to the sway of other civs. You can affect Loyalty through proximity--a city on the edge of your borders will be vulnerable to the charms of a nearby foreign city and vice versa--city projects, espionage, and more. Colonizing a separate continent requires more of a cost-benefit analysis than ever, as the danger of low Loyalty can outweigh the advantages of settling new regions. But you can also disrupt an opponent's Loyalty for your own gain, including the city itself (without suffering a Warmonger penalty).
Keeping your Loyalty high is more passive than it sounds thanks to various Loyalty-boosting improvements as well as Governors, new characters that you can gradually unlock and station in your cities. In addition to increasing Loyalty, each of the seven Governors has a different specialization (commerce, war, science, and so on) and can be leveled up to provide various benefits to the city they govern. They can be reassigned at any time, too, so you're not locked into one city being all culture-focused until the end of time. Most importantly, Governors are a further incentive to invest in each of your cities and consequently develop a more balanced civilization--not just one that can crank out science points until you win the space race in the 1800s.
Rise and Fall brings with it a series of smaller tweaks to round out the big additions. You can now form different kinds of alliances with other civs, like economic or military ones, so you can trade comfortably without going to war; there are also "Emergencies," triggered by things like taking over city-states and dropping nukes, during which civs can band together to address the threats and reap the benefits if they succeed. On top of that, there are new policies and small changes to the highly customizable government system, which means more options to tailor your government to your playstyle as it develops.
Of course, there are also new civs and leaders, which range from the battle-ready Genghis Khan of Mongolia to the science-minded Seondeok of Korea. There are nine new leaders but eight new civilizations; Chandragupta has been added as an alternative to Gandhi in India. The new civs aren't so much groundbreaking as they are a nice change of pace for Civ VI veterans who are eager to try out something new.
Unfortunately, Rise and Fall doesn't appear to improve the AI inconsistencies present in vanilla Civ VI. Some AI-controlled civs still act almost randomly--Japan declared war on me twice in one game despite never sending its military my way--while others are a bit more clever, declaring preemptive wars or offering strategic trades at opportune times. And while the Loyalty and Governors systems enhance city management and encourage you to pursue a wider variety of specialties than just your intended victory condition, religion remains the least dynamic of the avenues without anything in Rise and Fall drastically changing it.
As Civ VI's first expansion, though, Rise and Fall works so well with the base game that lingering issues are minor. It enhances, rather than overcomplicates, systems that were already deep and layered to begin with, while introducing features that keep each game engaging from start to finish. Ages in particular provide room for struggling civs to climb the ranks in the late game and keep leading civs on their toes, and the Governor and Loyalty systems add to the city-specific strategies that helped make the base game great.