Thank you for your suggestions. Here are some more sentences for you to check.
1)The inner station consisted of a long building standing (or which stood?) at the top of a hill. There was no fence around it but there had been one.
2)There were also some wooden posts standing around the building, which were decorated by round balls. After a while Marlow realized that the balls were not ornaments but human skulls, whose faces were turned towards the house.
3)A man on the riverbank waved at them. He looked like a harlequin in his highly coloured clothes. He was a Russian who decided to become a trader and began to trade in ivory. He told Marlow that Kurtz had enlarged his mind.
4)He told Marlow that Kurzt was so brilliant and talked so well. Though he disapproved of the skulls Kurzt used as ornaments to decorate his house, he had no courage to take them away.
5) Marlow didn’t admire his devotion to Kurtz.
6)Marlow could not decide what Kurzt’s real talent were. He was a sort of universal genius.
1. "standing" "... around it, but there...."
2. "... with round balls. After a while, Marlow...."
3. "... Russian, who...." "Enlarged" might not be the best term. See Thesaurus.
4. Can get rid of 2 "so"s. "... take them down." "Away" makes it seem like he might keep the skulls in another location.
5. Unclear referent.
6. "talents were" or "talent was." Delete "sort of." Can use "seemed like."