Rome was the civilized ruling centre of all the known world at this time.
The Gospel had made its way there in the hearts and out of the mouths of unknown Christians both Jew and Gentile. Primarily these people were of lowly and humble circumstance. Slaves, workers & merchants all trying to live in Rome the great city some there by choice some against their will.
Paul wrote to the church in Rome to share with them the central themes of the gospel and how those themes should be lived out. Paul had hoped to visit them and from there extend his missionary journey into Spain, unfortunately, his arrest and imprisonment prevented this.
Romans is a letter spelling out in detail the message he had hoped to share with them personally.