- "Kilwa is one of the most beautifully built cities in the world; the houses there are entirely made of wood, their rooftops out of rope grass, and it rains with great vigour."
– Ibn Battuta
Kilwa Kisiwani is an interestingly unique wonder, for it's the only wonder whose bonuses are entirely dependent on what size of map you are playing on. Unlike wonders such as the Pyramids, the Colosseum, the Mausoleum at Halicarnassus, or the Ruhr Valley, whose bonuses are static and consistently powerful across all games, Kilwa Kisiwani's bonuses to your civilization will ebb and flow even within a single game depending on what city-states are present and whether you gain (or lose) Suzerainty over them.
That said, Kilwa Kisiwani does not pale in comparison to any of these wonders, and given that you will have Suzerainty over at least a couple of city-states, it is always worth attempting to build. This wonder has very loose placement restrictions, so it should be relatively easy to build. The wonder's description makes it hard to visualize its power. For example, if you are the Suzerain of one industrial city-state, the city that builds this wonder will gain a 15% bonus Production; if you are the Suzerain of two or more industrial city-states, every single city in your empire will gain a 15% bonus Production (which is almost like having a Ruhr Valley in every city), and the parent city of Kilwa Kisiwani will get a whopping 30% bonus Production. It's crazy to think that the Ruhr Valley, at 1240 Production, is almost twice as expensive as this wonder without having the game-breaking, empire-wide potential. Likewise, if you have Suzerainty of two scientific city-states, that's almost the amount of Science you get from the Amundsen-Scott Research Station, as every city will receive 15% bonus Science and the parent city 30% - only this comes four eras sooner and avoids the hassle of having a Snow city.
With Rise and Fall (and by extension Gathering Storm), Amani and the Fabricate Scandal mission gives you alternatives to earning Envoys and shifts the balance of power; you are no longer bound to the civic tree - leaders with additional abilities to generate Envoys outside the civic tree (Pericles and Gorgo, Tamar, and Matthias Corvinus) still have a slight edge, but are no longer unbeatable in city-state diplomacy. As a result, Kilwa Kisiwani's bonuses are indirectly buffed, and it becomes more imperative to construct this wonder.
To sum it up, this is a ridiculously powerful wonder that flies under the radar of most players. The larger the map is, the more city-states there are, and the more likely you are to get Suzerainty of two city-states of the same type, the more powerful this wonder grows. All the wonders that grant a flat percentage of yields (Ruhr Valley, Oxford University, Broadway, Kotoku-in) grant only 20% of their respective yields to the parent city; at its full potential (which is relatively simple to achieve on a Large or Huge map), Kilwa Kisiwani grants 30% yields to its parent city and 15% empire-wide, and it can grant as many types of yields as you can gain Suzerainty of. It is also cheaper and available earlier than almost all of those wonders mentioned above (except for Kotuku-in).
In the Barbarian Clans game mode, the ability of Barbarians to civilize makes Kilwa Kisiwani a must-have wonder, especially on larger maps. Late in the game, when all the outposts that spawned in remote areas have become city-states, its potential to help its owner generate Faith, Production, Science, Gold, and Culture is nothing short of astronomical.
The East African city-state Kilwa Kisiwani was once a major destination for trade in the Indian Ocean. Ivory, gold, and perfumes flowed through its markets. At its height, Kilwa Kisiwani contained some of the finest architecture in East Africa: the Great Mosque (which remained the world’s largest mosque until the 16th Century) and Husuni Kubwa (a classy palace), both of which survived to the modern day.
Kilwa Kisiwani flourished as a major trade city until the Portuguese arrived in 1498 to “make improvements” to the city’s infrastructure (if extorting tribute can be considered an improvement). Over the centuries, a series of foreign conquerors wrestled control of the city from one another, until it was abandoned in 1840.