Rome is the capital of Italy, a European country. There are a number of great buildings which were constructed hundreds of years ago.

When we say "Rome was not built in one day", we should not expect to build something great in a short period of time.

Example:

Son: "Uncle Tom told me that he owns a large factory with 6,000 workers in the Mainland."

Father: "Rome was not built in one day. It takes him many years and great efforts to become successful."