Tile yields and improvements

tile_yield(x, y, kind) in CITYMGT.C returns food (kind 0), shields (1) or trade (2). It first selects the base or special-resource record. It then applies these rules in order:

1. Irrigation and mining alter land yields using work_rules. A road adds one trade on Desert, Plains and Grassland.
2. The coordinate pattern can remove shields from Grassland and River.
3. The Colossus adds one to non-zero trade in its own city.
4. Railroad adds half the current yield, rounded down, to each category.
5. Anarchy and Despotism lose one from yields above two, unless the city is celebrating.
6. Non-zero trade gains one under Republic and Democracy. Celebrating cities receive this bonus from Monarchy upwards.
7. Pollution halves the result, rounded up. Negative results become zero.

The order matters: a railroad bonus is applied before the Despotism penalty and the final pollution reduction. Pollution reduces yields; it does not necessarily remove them.

Settler work

CONTROL.C handles road, railroad, irrigation, mining, fortress and pollution-cleaning commands. work_rules supplies terrain-specific work and transformation values. Some orders change the terrain type instead of simply setting an improvement bit. Work progress shares the unit’s fuel byte with aircraft range tracking.

Road and railroad movement rules are described in Movement. Rivers do not give road movement costs. A fortress doubles ordinary terrain defence; it does not add to a unit’s fortification bonus.