city building

This unit can build cities.

The building of cities and forts can make a permanent claim on land. The normal difficulty of building something is 4 but the difficulty of the area is added to this number.
When two units with the same level of city building work together they can be treated like one unit with one point extra in city building.
Also if a unit works double the turns on the same project it counts like an extra point of city building.

A small example:
The player has 3 units: two engineers and a craftsmen unit. The engineers have city building 2 and the craftsmen unit has city building 3. They build a city at the coast that makes it one point more difficult to build a city.
The skill of the two engineers can be combined to a skill of 3.
The skill of all three units is 4.
When they work 2 turns their combined skill is 5 that is enough to build this city.