A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
jos 20:1THE LORD COMMANDS THE CITIES OF REFUGE. (Jos 20:1-6)
The Lord spake unto Joshua . . . Appoint out for you cities of refuge--(See Num. 35:9-28; Deu 19:1-13). The command here recorded was given on their going to occupy their allotted settlements. The sanctuaries were not temples or altars, as in other countries, but inhabited cities; and the design was not to screen criminals, but only to afford the homicide protection from the vengeance of the deceased's relatives until it should have been ascertained whether the death had resulted from accident and momentary passion, or from premeditated malice. The institution of the cities of refuge, together with the rules prescribed for the guidance of those who sought an asylum within their walls, was an important provision, tending to secure the ends of justice as well as of mercy.
jos 20:4he that doth flee unto one of those cities shall stand at the entering of the gate of the city--It was the place of public resort, and on arriving there he related his tale of distress to the elders, who were bound to give him shelter and the means of support, until the local authorities (Jos 20:6), having carefully investigated the case, should have pronounced the decision. If found guilty, the manslayer was surrendered to the blood-avenger; if extenuating circumstances appeared, he was to remain in the city of refuge, where he would be safe from the vindictive feelings of his pursuers; but he forfeited the privilege of immunity the moment he ventured beyond the walls.
jos 20:6until the death of the high priest--His death secured the complete deliverance of the manslayer from his sin, only because he had been anointed with the holy oil (Num 35:25), the symbol of the Holy Ghost; and thus the death of the earthly high priest became a type of that of the heavenly one (Heb 9:14-15).
jos 20:7THE ISRAELITES APPOINT BY NAME THE CITIES OF REFUGE. (Jos 20:7-9)
they appointed . . . cities--There were six; three on the west, and three on the east, of Jordan. In the first instance, they were a provision of the criminal law of the Hebrews, necessary in the circumstances of that people (see on Num 35:11; Deu 19:2). At the same time they were designed also typically to point out the sinner's way to Christ (Heb 6:18).