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
deu 19:2OF THE CITIES OF REFUGE. (Deu 19:1-13)
Thou shalt separate three cities for thee in the midst of thy land--Goelism, or the duty of the nearest kinsmen to avenge the death of a slaughtered relative, being the customary law of that age (as it still is among the Arabs and other people of the East), Moses incorporated it in an improved form with his legislative code. For the protection of the unintentional homicide, he provided certain cities of refuge--three had been destined for this purpose on the east of Jordan (Deu 4:41; Num 35:11); three were to be invested with the same privilege on the west of that river when Canaan should be conquered.
in the midst of thy land--in such a position that they would be conspicuous and accessible, and equidistant from the extremities of the land and from each other.
deu 19:3Thou shalt prepare thee a way--The roads leading to them were to be kept in good condition and the brooks or rivers to be spanned by good bridges; the width of the roads was to be thirty-two cubits; and at all the crossroads signposts were to be erected with the words, Mekeleth, Mekeleth, "refuge, refuge," painted on them.
divide the coasts of thy land . . . into three parts--the whole extent of the country from the south to the north. The three cities on each side of Jordan were opposite to each other, "as two rows of vines in a vineyard" (see on Jos 20:7).
deu 19:6Lest the avenger of the blood pursue the slayer, while his heart is hot--This verse is a continuation of Deu 19:3 (for Deu 19:4-5, which are explanatory, are in a parenthetical form), and the meaning is that if the kinsman of a person inadvertently killed should, under the impulse of sudden excitement and without inquiring into the circumstances, inflict summary vengeance on the homicide, however guiltless, the law tolerated such an act; it was to pass with impunity. But to prevent such precipitate measures, the cities of refuge were established for the reception of the homicide, that "innocent blood might not be shed in thy land" (Deu 19:10). In the case of premeditated murder (Deu 19:11-12), they afforded no immunity; but, if it were only manslaughter, the moment the fugitive was within the gates, he found himself in a safe asylum (Num 35:26-28; Jos 20:6).
deu 19:8And if the Lord thy God enlarge thy coast--Three additional sanctuaries were to be established in the event of their territory extending over the country from Hermon and Gilead to the Euphrates (see Gen 15:18; Exo 23:31). But it was obscurely hinted that this last provision would never be carried into effect, as the Israelites would not fulfil the conditions, namely, "that of keeping the commandments, to love the Lord, and walk ever in his ways." In point of fact, although that region was brought into subjection by David and Solomon, we do not find that cities of refuge were established; because those sovereigns only made the ancient inhabitants tributary, instead of sending a colony of Israelites to possess it. The privilege of sanctuary cities, however, was given only for Israelites; and besides, that conquered territory did not remain long under the power of the Hebrew kings.
deu 19:14THE LANDMARK IS NOT TO BE REMOVED. (Deu 19:14)
Thou shalt not remove thy neighbour's landmark, which they of old have set in thine inheritance--The state of Palestine in regard to enclosures is very much the same now as it has always been. Though gardens and vineyards are surrounded by dry-stone walls or hedges of prickly pear, the boundaries of arable fields are marked by nothing but by a little trench, a small cairn, or a single erect stone, placed at certain intervals. It is manifest that a dishonest person could easily fill the gutter with earth, or remove these stones a few feet without much risk of detection and so enlarge his own field by a stealthy encroachment on his neighbor's. This law, then, was made to prevent such trespasses.
deu 19:15TWO WITNESSES REQUIRED. (Deu 19:15)
One witness shall not rise up against a man for any iniquity--The following rules to regulate the admission of testimony in public courts are founded on the principles of natural justice. A single witness shall not be admitted to the condemnation of an accused person.
deu 19:16PUNISHMENT OF A FALSE WITNESS. (Deu 19:16-21)
But if convicted of perjury, it will be sufficient for his own condemnation, and his punishment shall be exactly the same as would have overtaken the object of his malignant prosecution. (See on Exo 21:23; see also Lev 24:20).