|Amidah-Sim Shalom-A Place of Peace-Ramban|
The 11th of Nisan is the Yahrtzeit of Rav Moshe ben Nachman, the Ramban: Rabbi Abraham said, lest particles of the hewn stone will end up in refuse, and the other parts will be used to build an altar for God; or one may use them to build an altar for idolatry, because it's worshipers do so hoping to be successful. Rambam said in his Guide for the Perplexed (3:45), the purpose is not to come to make an image, and it be a “bowing down” stone. That was the custom of the idol worshipers.
But I say, the reason for this mitzvah is because iron is the sword that destroys the world. That is, its name; Herev (sword) and Hurban (destruction) have the same root. And, behold, Esau, who is hated by God, he is the inheritor of the sword, as it is written, “By your sword shall you live (Genesis 27:40),” and the sword is his power in heaven and on earth, for under the influence of Mars and the constellations of blood, the sword succeeds, and under them it shows its strength, therefore, it must not be brought into the House of God.
That is the reason that the verse says explicitly, “You shall not build it of hewn stone,” for by swinging over them any iron to shape them, you have swung over it your sword that murders and increases casualties and consequently desecrated the altar. Therefore, there was no iron in the Tabernacle, for even its pegs, which would have been better of iron, he made of copper, and thus in the Temple.