Each level after level 20 in Sydney can be played three times with each subsequent tier becoming more challenging.
The three difficulty modes in ascending order are Intern, Architect, and Master Builder.
To complete a city and unlock the next, each level’s Intern mode must be beaten once. A level's Architect and Master Builder modes can be played to earn more coins, more resources, or other rewards like purple Local Workers.
The final tier of a level can be replayed an unlimited amount of times, but doing so will not earn gold coins.