Livy on the Roman Senate
During the regal period, King Tullus Hostilius built the Curia Hostilia, the Roman Senate building.
Because it was a templum -- a consecrated place, it was oriented north/south, like other temples in Rome.
The first king, Romulus, created the original Senate to serve as an advisory staff. Later, the senators' powers became extensive, although they did not write laws: Senators handled treaties, alliances, war, and more. During the early period their numbers increased from 100-300. Once senators assumed office, they were there for life, unless kicked out. Once a Roman held a magisterial office, he became a senator. Thus, insofar as the magistracies were elected offices, senators were elected, but they were not directly elected to be senators.
Read relevant passages on the ancient history of the Roman Senate and Senators from Livy.