Between 600,000 and 200,000 years ago, "Heidelberg Man" died at nearby Mauer. His jaw bone was discovered in 1907; with scientific dating, his remains were determined to be the earliest evidence of human life in Europe. In the 5th century BC, a Celtic fortress of refuge and place of worship were built on the Heiligenberg, or "Mountain of Saints". Both places can still be identified. In 40 AD, a fort was built and occupied by the 24th Roman cohort and the 2nd Cyrenaican cohort (CCG XXIIII and CCH II CYR). The Romans built and maintained castra (permanent camps) and a signalling tower on the bank of the Neckar. They built a wooden bridge based on stone pillars across it. The camp protected the first civilian settlements that developed. The Romans remained until 260 AD, when the camp was conquered by Germanic tribes.
Modern Heidelberg can trace its beginnings to the fifth century. The village Bergheim (Mountain Home) is first mentioned in that period, in documents dated to 769 AD. Bergheim now lies in the middle of modern Heidelberg. The people gradually converted to Christianity. In 863 AD, the monastery of St. Michael was founded on the Heiligenberg inside the double rampart of the Celtic fortress. Around 1130, the Neuberg Monastery was founded in the Neckar valley. At the same time, the bishopric of Worms extended its influence into the valley, founding Sch�nau Abbey in 1142. Modern Heidelberg can trace its roots to this 12th-century monastery. The first reference to Heidelberg can be found in a document in Sch�nau Abbey dated to 1196. This is considered the founding date for Heidelberg. In 1155, Heidelberg castle and its neighboring settlement were taken over by the house of Hohenstaufen. Conrad of Hohenstaufen became Count Palatine of the Rhine (German: Pfalzgraf bei Rhein). In 1195, the Electorate of the Palatinate passed to the House of Welf through marriage.
In 1225, Louis I, Duke of Bavaria obtained the Palatinate, and thus the castle came under
