It is possible to gain resources by exploiting cheaper unit keep influence as the French. This is caused by the fact that resource returns upon unqueueing units are determined by the current price of the unit rather than the price at queueing time. The steps are simple:
- Build a keep as the French.
- Build a stable/archery range in the keep’s influence area.
- Queue a unit with 20% (or 25% after the upgrade, its tooltip is bugged and applies -5%, not -10%) cost reduction, spending 80% resources.
- Delete the keep while the unit is in the queue; this will remove the keep’s influence from the production building.
- Unqueue the unit, and 100% of the unit cost will be returned rather than the 80% you’ve just spent.
Let’s say we have the upgrade Enlistment Incentives and are making royal knights 25% cheaper, for the cost of 105f/75g instead of the standard 140f/100g.
- Have 4+ stables around a keep, which means up to at least 60 royal knights can be queued.
- Queue 50 royal knights for the cost of 50x105 = 5250 food and 50x75 = 3750 gold.
- Delete the keep while the knights are in production.
- Unqueue all 50 royal knights. This will return their full cost of 50x140 = 7000 food and 50x100 = 5000 gold.
- A net gain of 7000 - 5250 = 1750 food and 5000 - 3750 = 1250 gold is achieved out of thin air with the only cost being a deleted keep. This is just a mild example and can get much worse with longer unit queues.
The reverse also works: If you queue a stable/archery range unit at full cost, then build a keep near it and then unqueue; you will only get part of the cost back, making your resources disappear.
This could also happen with similar buildings like the Rus High Armory or Chinese Spirit Way upon being destroyed, but those cannot be deleted by the player since they are landmarks, which would make it hard to exploit. I have not tested those yet.