The Feudalism wave is a timing tactic in Civilization 6. It is to use Ilkum policy (+30% Production for Builders) to have every city make a Builder just before unlocking the replacement policy, Serfdom (+2 uses for new Builders). When executed correctly this allows you to take advantage of the production bonus when making the Builders and get the extra charges out of them. I'm going to explain how set up the play and what its strategic ramifications are both before and after the wave. Setting up the wave In order to maximize the impact of the wave you'll want to have every city start progressing on a Builder when you start going for Feudalism. Cities with less production will need to start earlier while more developed cities may be able to fit in a few other things before switching to a Builder. In the likely event that the completion turn for the Builders are not perfectly aligned you can work on them until there is only a single turn left and then switch the city to another task. Once Feudalism is complete and Serfdom is adopted you can switch the cities back to their Builder and have it complete. You want the Builders to complete the turn after you get Feudalism (so that you can have Serfdom assigned) not the turn you get Feudalism. Boosting Feudalism The inspiration for Feudalism is: Build 6 Farms. To achieve this you'll need two previously created Builders. The best way to get the Farms is usually as two triangle sets of three. That way the clusters will be ready to receive the bonus food provided by Feudalism. If you have Rice or Wheat near you at the start of the game, those resources are useful for more than triggering Irrigation's boost. They are natural locations for your triangle clusters of farms. Keep these potential triangle clusters in mind when placing districts. What does this get you? Some numbers will help give context to how efficient this tactic is. Assuming a Builder costs 60 Production, how much Production per use do you get in various circumstances? Nothing: 60 Production/3 uses: 20 per use With 30% boost: 60/1.3=46 Production/3 uses: 15 per use With +2 uses: 60 Production/5 uses: 12 per use With 30% boost & +2 uses: 60/1.3=46 Production/5 uses: 9 per use Builders created during the wave are 33% (12/9) more efficient than Builders created with just Serfdom alone. And 66% (15/9) more efficient than Builders created with just Ilkum. Riding the Wave Now that we have a bunch of efficiently constructed Builders, how will we take advantage of them? Fortunately, Feudalism has an answer for us! Build triangle farm clusters. These will help rapidly grow a city's population, allowing it to work more tiles which can be improved by your Builders. Extra charges can also be used to chop features or harvest resources to make room for triangle farm clusters or new districts. After Feudalism is complete it is a good idea to grab a cheap Civic so that you can quickly switch out of Serfdom and back into another policy. The advantage of the wave is that you only need to run Serfdom for one or two turns while your queued up Builders complete before switching out. Strategic Ramifications If you plan to do the wave there are additional subtle things you can do before hand to make it more profitable. If few of your tiles need improvement you can do a wave of expansion before preparing for the wave. Depending on their initial production these newly founded cities may even be able to contribute workers to your wave. An expansion wave would require spending a some turns with the Colonization policy (+50% Production for Settlers) before switching over to Ilkum. Those Settlers would need protection though so prior to the Settler wave spending some time cranking out Archers to protect them would be a good idea. All together this forms three sequential waves that end where the next one begins. An Archer wave, followed by a Settle wave, followed by a Builder wave.