Alrighty, on Polder yields. I am working under these assumptions:
- Polders should be competitive with farms on food. That is, they should equal a good farm but not the best farm. If you have to choose between them frequently and it's a hard choice, then it's not a unique bonus, it's a trade-off. We don't ask trade-offs for unique buildings and units, the same should apply to unique improvements.
- Polders can be built on marsh (keeping the 3 food) and adjacent to freshwater (which naturally includes next to lakes and on flood plains). They clear forests and jungles, and cannot be built on hills (doesn't work graphically or thematically)
- We want to encourage the Dutch build villages on roads next to polders, without encouraging maximum villages next to polders
- 'Flat, singular boost' assumes it will give the boost from the moment you have the improvement, and the boost can only effect one yield.
Base 3 Food 1 Gold, adjacent Villages +1 Food
with Economics +2 Gold
with Chemistry +1 Culture
Unless terrain gen goes crazy with rivers and lakes (which it does sometimes, this Oval I'm looking at is more river than not), you're unlikely to get more than 3 polders adjacent to a village, and 3 will be unusual itself. On Grassland, at Chemistry, this gives you 3 tiles of 5F 3G 1C, and 1 tile of 5F 2P 4G 1C. If you had built farms, you would have 4 tiles of 5F. The polder arrangement is giving the same food in a pretty good terrain situation, along with the cash, culture, and production.
If you were to replace a Polder with a Village not on a road, you would need 3 Polders adjacent to the village to break even. Village provides 2G 1C by default. On grassland with +3 food from the polders, it's a 5F 2G 1C tile. Good, but not as good as putting a polder there instead. Because polders must be build on freshwater, it is rather rare that they will be able to build the 'perfect' layout of 6 polders around 1 village. In the case that they do on Grassland, and you build your road and send your traderoute over your village, literal PERFECT conditions, you have 6 tiles of 5F 3G 1C and 1 tile of 8F 2P 4G 1C. Seven farms in the same situation would provide 6 tiles of 5F and 1 tile of 7F.
As for the terrace farm, I have a less solid opinion. I feel they should unlock at Construction, have another food base (2F 1P 1C), and provide +1F at Engineering. However, all of this is mainly thematically driven without deeply examining the mechanical balance implications. I have yet to play Inca in CBP. As far as adjacency goes, Food probably makes the most sense. Production or Culture could work.