The tale of Romulus and Remus is one of the most famous myths in World History, and most historians agree that it is a legend, and there is very little historical evidence to support any other theory.
Remus and Romulus were the sons of the daughter of Alba Longa's king. Alba Longa is a real place and tribes have inhabited it for a long time, so if there is any historical fact invovled in the story it most likely exists there. Their father was apparently Mars, and eventually the two twins grew up and their natural leadership skills won them many followers.
The two brothers wanted to found a city, but disagreed on the location. Romulus killed his brother during the argument and founded his city, Rome, on the Paletine Hill. This is widley considered the oldest part of Rome. He eventually founded the Legions, Senate, and kidnapped the women from the Sabine tribe to populate his city. There is actually some written and archelogical evidence to support this since Rome and the Sabines were at war in 505 B.C.