Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
The king was thus simply an ordinary burgess, whom merit or fortune, and the primary necessity of having one as master in every house, had placed as master over his equals--a husbandman set over husbandmen, a warrior set over warriors. As the son absolutely obeyed his father and yet did not esteem himself inferior, so the burgess submitted to his ruler without precisely accounting him his better. This constituted the moral and practical limitation of the regal power.
The king might, it is true, do much that was inconsistent with equity without exactly breaking the law of the land: he might diminish his fellow-combatants' share of the spoil; he might impose exorbitant task-works or otherwise by his imposts unreasonably encroach upon the property of the burgess; but if he did so, he forgot that his plenary power came not from God, but under God's consent from the people, whose representative he was; and who was there to protect him, if the people should in return forget the oath of allegiance which they had sworn? The legal limitation, again, of the king's power lay in the principle that he was entitled only to execute the law, not to alterit.
Every deviation from the law had to receive the previous approval of the assembly of the people and the council of elders; if it was not so approved, it was a null and tyrannical act carrying no legal effect. Thus the power of the king in Rome was, both morally and legally, at bottom altogether different from the sovereignty of the present day; and there is no counterpart at all in modern life either to the Roman household or to the Roman state.
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Reference address : https://www.ellopos.net/elpenor/rome/1-05-original-constitution-rome.asp?pg=13