A man needed hospital treatment for a broken nose after being assaulted in town.
The victim was punched repeatedly and suffered cuts to his face at around 2am on Sunday 5 February.
It happened between the Peirson pub and Thompson Estates, in the alley leading from King Street to the Royal Square.
The offender was with another man.
Both have been described being in their early 20s and one was wearing a white polo with dark blue jeans, a black gilet and black trainers.
Police are looking for witnesses.
They say the victim was with a woman who was wearing a grey fitness style ¾ zip tip, dark jeans and white trainers.
An 18-year-old man has been arrested on suspicion of grave and criminal assault and has been released on bail as investigations continue.
Anyone who has any information can contact police on 612612 or Crimestoppers anonymously on 0800 555 111 or via their Anonymous Online Form at Crimestoppers-uk.org.