Plutarch Romulus 11

[1] Romulus buried Remus, together with his foster-fathers, in the Remonia, ¹ and then set himself to building his city, after summoning from Tuscany men who prescribed all the details in accordance with certain sacred ordinances and writings, and taught them to him as in a religious rite. A circular trench was dug around what is now the Comitium, ² and in this were deposited first-fruits of all things the use of which was sanctioned by custom as good and by nature as necessary; and finally, every man brought a small portion of the soil of his native land, and these were cast in among the first-fruits and mingled with them.

[2] They call this trench, as they do the heavens, by the name of mundus. Then, taking this as a centre, they marked out the city in a circle round it. And the founder, having shod a plough with a brazen ploughshare, and having yoked to it a bull and a cow, himself drove a deep furrow round the boundary lines, while those who followed after him had to turn the clods, which the plough threw up, inwards towards the city, and suffer no clod to lie turned outwards.

[3] With this line they mark out the course of the wall, and it is called, by contraction, pomerium, that is post murum, behind or next the wall. And where they purposed to put in a gate, there they took the share out of the ground, lifted the plough over, and left a vacant space. And this is the reason why they regard all the wall as sacred except the gates; but if they held the gates sacred, it would not be possible, without religious scruples, to bring into and send out of the city things which are necessary, and yet unclean.