Romulus and Remus were twin brothers of Mars their father who is the God of War, and Rhea Silvia their mother who is the daughter of King Numitor.
Amulius the uncle of Rhea Silvia was greedy and wanted to be the king, so he grasped the twin brothers leaving them in a basket and threw them into the River Tiber hoping that they would drown.
The twin brothers were in luck and were rescued by a female wolf who fed her own milk. Then, they were found by a shepherd Fastulus that took care of them.
The twin brothers discovered who they really were over time, they killed Amulius and put their grandfather back on the throne.
They each build their own city with walls, Remus build it in Aventine Hill and Romulus build it in Palatine Hill. Remus made fun of how the wall that Romulus build can be easily breached which made Romulus kill him.
Romulus populated his country with runaway slaves and criminals. He also provided wives for them to populate faster. This is the myth of how Rome was build.