Adam Clarke on Numbers 35:12

Until he stand before the congregation in judgment - So one of these cities was not a perpetual asylum; It was only a pro tempore refuge, till the case could be fairly examined by the magistrates in the presence of the people, or the elders their representatives; and this was done in the city or place where he had done the murder, Jos 20:4, Jos 20:6. If he was found worthy of death, they delivered him to the avenger that he might be slain, Deu 19:12; if not, they sent him back to the city of refuge, where he remained till the death of the high priest, Num 35:25. Before the cities of refuge were appointed, the altar appears to have been a sanctuary for those who had killed a person unwittingly; see on Exo 21:13 (note), and Exo 21:14 (note).