The leveling up of croplands is slightly different from other resource fields because when your cropland reaches L5, you get access to the Grain Mill. Furthermore, you get access to the Bakery at L10.
Grain Mill and Bakery each provide a 5 percent boost in crop production for each level it is upgraded (maximum level 5). The boost is cumulative, so a fully upgraded Grain Mill and Bakery (both level 5) provides a 50% increase in crop production.
The boost is calculated on a field level and rounded up to the nearest number. For example, a L6 cropland produces 50 crop. With a Grain Mill L1 boost, it increases production by 5 percent. That makes 50 x 5% = 2.5 crop. Rounded up, each L6 cropland produces 53 crop. So a 6 cropper village produces 6 x 53 = 318 crop.
The cost of a Grain Mill/Bakery is quite expensive however and may not yet be worth the price. When it is worth the price also depends whether it is a 6, 9 or 15 crop village.
However, as a general rule of thumb, the following is a recommended build order for a regular 6 cropper village:-
- Upgrade all croplands to L6.
- Build Grain Mill L1.
- Upgrade all croplands to L7.
- Upgrade Grain Mill to L2.
- Upgrade all croplands to L8.
- Upgrade Grain Mill to L3.
- Upgrade one cropland to L10.
- Upgrade Grain Mill to L5.
- Build Bakery L1, L2 and L3.
- Upgrade remaining croplands to L9.
- Upgrade Bakery to L4.
- Upgrade remaining croplands to L10.
- Upgrade Bakery to L5.
For 9 and 15 croppers, build the Grain Mill and Bakery about 1 level earlier.