The body of 25-year-old Conall Kerrigan from Claudy was discovered in Bank Place at around 22:20 BST on Sunday.
The police are treating his death as "unexplained."
They have appealed for anyone who may have seen him after he left the Metro Bar early on Sunday morning to contact them.
They also want to hear from people who may have noticed a fight on the city walls near the Millennium Forum.
The 25-year-old's cousin Aaron Kerrigan said he looked on him as a brother.
"If you look at his photos - just every single one of them he's smiling," he said.
"That's what he epitomised. He could bring a smile to anybody's face just by a look or a word.
"He was somebody I looked up to. It's just really difficult."
The family of a man found dead in County Londonderry last night have said they are devastated at his death.