IGG has released an overview of Guild Built Cities and how these cities are leveled up in Voyage Century Online.

With the new version of VCO test approaching the end, the City-Building System update will be released soon. The officials have already announced the City-Building System update rules.

How can a Guild Built City level up?

After reaching the required leveling index, each guild member can find the [Construction Minister] and pay the appointed leveling fund and provide the required leveling materials to meet the requirements. When the guild meets the leveling material requirements, the Leader or Vice Leader can level up the port. The port will obtain one [Development Point] after each level. Meanwhile, there is a Cooling Time before the next leveling.

Leveling Rules:

1. Meet the development index requirements.
2. Meet the leveling requirements.
3. Requires a Cooling Time before leveling again.
4. Only the Leader or Vice Leader can level up the port.
5. The port can be leveled up when the Residual Leveling Time is 0. (The system will give a prompt to the guild leaders about the residual leveling time before the next leveling.)

Read more here.