Commentaire de citation sur Le Deutéronome 27:5

When you enter After previously saying “when God grants you rest” [25: 19], God now states that there are other commandments which apply before “God grants you rest” — in fact, as soon as you enter the land — namely, first-fruits [: 2 ff], tithes [: 12 ff], the writing of the Torah on stones [27: 2 ff], the building of an altar [27: 5 ff], and the recitation of the blessing and the curse upon the two mountains [27: 12 ff].

There is also an allusion in the stone altar, that the stones should be whole and not hewn; similarly, a judge should be perfect, without any blemish, without additions or subtractions, and should not need external beautification or adornment like a hewn stone upon which a sword has been raised, profaning it from its sanctity. The same reasoning applies to a judge who needs external correction, which gives place for the axe and the hewer to boast saying, “I appointed you, and on your own merit you are not worthy.” Therefore, it is good to choose whole stones that do not need external repair.
Rashi explained that one can derive a kal vachomer [a fortiori argument] from stones, which neither see nor hear, etc., and because they bring peace, the Torah said You shall not raise iron upon them (Deuteronomy 27:5). One who brings peace between a man and his wife, or between a person and his fellow, how much more so will no calamity befall him.
It seems that Rashi’s intention with this kal vachomer is to give a reason for the proximity of the Sanhedrin to the altar, because anyone who sits in judgment brings peace between a person and his fellow. And lest the judge fear anyone, that the one found guilty in judgment may pursue him, and the judge might want to remove himself from judgment to avoid harm, therefore the Sanhedrin sat near the altar, so they would learn a kal vachomer from the altar that no harm would befall them. And this is a precious interpretation.

“And you shall make for it a grate of netting of copper.” Because the evil inclination spreads a net to capture a person in its trap, and the net of the altar removes him from the net of his evil inclination. We find that there were miracles with the altar, for it was saved from fire, as the fire burned on it day and night yet the fire did not burn it nor melt its copper. It was saved from water, as the rains did not extinguish the fire of the arranged wood. It was saved from wind, as the wind did not overpower the column of smoke. The altar was hollow and filled with earth, or it was an altar of stones upon which no iron tool was raised. All this alludes to a person, that through the altar, a person is saved and does not receive damage in all four elements within him. As it is written: We went through fire and through water, yet You brought us out to abundance. I will enter Your house with burnt offerings (Psalms 66:12–13). For just as the altar of burnt offerings is saved from fire and water, so too will a person be saved from them through it.
And likewise, the altar saves him from all four death penalties of the court: from stoning with stones he is saved through the stone altar (Deuteronomy 27:5); from burning with fire he is saved through the fire of the altar which burns on it (Leviticus 6:5); from execution by sword he is saved through the altar, about which it is said (Deuteronomy 27:5), Do not raise an iron tool on them; from strangulation he is saved, in the way that our Sages of blessed memory said (Ketubot 30b), “Even though the four death penalties of the court have been abolished, nevertheless the judgment of the four death penalties has not been abolished. One who is liable to strangulation either drowns in a river, etc.” It follows that just as the altar is saved from the flood of rainwater, so too the sinner — to the flood of many waters, they will not reach him (Psalms 32:6).

They brought the stones - as is written "there, too, you shall build an altar ... and on [those stones] you shall inscribe every word of this Teaching most distinctly" [Deuteronomy 27:5,8]