Why does Conrad speak about the Roman conquest in Heart of Darkness and why is Marlow is compared to Buddha?
Conrad's reference to the Roman conquest of Britain is allegorical. Marlow's like a Roman soldier travelling to the very edge of the known world, exploring a strange new land, dark and exotic. He reflects on what must have been going through the soldiers' minds when they first encountered ancient Britain, and all the various hardships they had to endure—lousy weather, precious little to eat, and being forced to drink water from the polluted Thames.
Yet Marlow understands that there are differences between himself and the Roman soldiers. For one thing, his experiences have made him more reflective, more philosophical about things. Hence his being compared to Buddha. The Romans were driven by the desire for conquest in their exploration of Britain; Marlow initially saw his own colonial adventure as having a more noble motivation, one characterized by an unselfish belief in a higher cause. However, Marlow soon pauses as he recounts his tale, remembering how thoroughly disillusioned he had become with the entire colonial project by the end of his terrifying adventure.
At the beginning of the story, Marlow talks about the Roman conquest of England and what the area that is now London must have been like before the Romans came. He does this to make an analogy -- he is talking about how England used to be barbaric and comparing it to Africa in his own day. He is saying his journey in Africa was like that of a Roman soldier to England.
Marlow is compared to Buddha because his journey to Africa and his meeting with Kurtz has enlightened him. He knows more about the human soul than he did before.