Okay, so imagine you live in a big town with a lot of people and a lot of buildings. One day, a bunch of really smart people get together and decide that they want to make the town even better. They come up with a plan to build a whole new city right next to the old town.
This new city is going to have wide streets, big parks, and lots of cool buildings. They decide to name it the City of Adelaide, after a queen who was very important at the time. But before they can start building, they have to make a plan.
They draw a big map of the new city and decide where everything should go. They put the most important building, like the government ones, in the middle. Then, they make sure that all the streets are really wide, so they can fit lots of people and carriages (because there were no cars back then).
They also decide to plant a lot of trees and make big parks where people can play and have picnics. They want the new city to be really nice to live in. When they finally start building, everything looks so pretty and organized.
Once the City of Adelaide is finished, lots of people move there because it's such a great place to live. And even though it's been a long time since it was built, it's still a really important city in Australia today.