In my copy* of Meditations the quote reads:
- Let this always be plain to you, that this piece of land is like any other; and that what is here is the same as what is on top of a mountain, or on the seashore, or wherever you choose; as Plato says of his philosopher, whose retreat is "like a shepherd's fold on a mountain."
...and has an annotation stating the quote is from Plato's Theaetetus. Specifically the quote from Theaetetus (as translated by M. J. Levett, revised by Myles Burnyeat) reads, "for the castle of the one is as much a prison as the mountain fold of the other."
To get at Aurelius's meaning it might help to read the preceding passages:
SOCRATES: Well, here’s an instance: they say Thales was studying the
stars, Theodorus, and gazing aloft, when he fell into a well; and a witty
and amusing Thracian servant-girl made fun of him because, she said, he
was wild to know about what was up in the sky but failed to see what
was in front of him and under his feet. The same joke applies to all who
spend their lives in philosophy. It really is true that the philosopher fails
to see his next-door neighbor; he not only doesn’t notice what he is doing;
he scarcely knows whether he is a man or some other kind of creature.
The question he asks is, What is Man? What actions and passions properly
belong to human nature and distinguish it from all other beings? This is
what he wants to know and concerns himself to investigate. You see what
I mean, Theodorus, don’t you?
THEODORUS: Yes, and what you say is true.
SOCRATES: This accounts, my friend, for the behavior of such a man when
he comes into contact with his fellows, either privately with individuals
or in public life, as I was saying at the beginning. Whenever he is obliged,
in a law court or elsewhere, to discuss the things that lie at his feet and
before his eyes, he causes entertainment not only to Thracian servant-girls
but to all the common herd, by tumbling into wells and every sort of
difficulty through his lack of experience. His clumsiness is awful and gets
him a reputation for fatuousness. On occasions when personal scandal is
the topic of conversation, he never has anything at all of his own to
contribute; he knows nothing to the detriment of anyone, never having
paid any attention to this subject—a lack of resource which makes him
look very comic. And again, when compliments are in order, and self-laudation,
his evident amusement—which is by no means a pose but perfectly genuine—is regarded as idiotic. When he hears the praises of a despot or a king being sung, it sounds to his ears as if some stock-breeder were being congratulated—some keeper of pigs or sheep, or cows that are giving him plenty of milk; only he thinks that the rulers have a more difficult and treacherous animal to rear and milk, and that such a man, having no spare time, is bound to become quite as coarse and uncultivated as the stock-farmer; for the castle of the one is as much a prison as the mountain fold of the other. ... On all these occasions, you see, the philosopher is the object of general derision, partly for what men take to be his superior manner, and partly for his constant ignorance and lack of resource in dealing with the obvious.
In particular, the "philosopher" Plato speaks of through his fictional character Socrates is Thales, "The first founder of Greek natural philosophy (sixth century B.C.), about whom we have anecdotes but little solid information." His point, tho is regarding the character of philosophers in general.
Also, to understand Aurelius's meaning, consider Bertrand Russell's comments upon Meditations from The History of Western Philosophy:
Marcus Aurelius ( A.D. 121-180) was at the other end of the social scale. He was the adopted son of the good Emperor Antoninus Pius, who was his uncle and his father-in-law, whom he succeeded in A.D. 161, and whose memory he revered. As Emperor, he devoted himself to Stoic virtue. He had much need of fortitude, for his reign was beset by calamities--earthquakes, pestilences, long and difficult wars, military insurrections. His Meditations, which are addressed to himself, and apparently not intended for publication, show that he felt his public duties burdensome, and that he suffered from a great weariness. His only son Commodus, who succeeded him, turned out to be one of the worst of the many bad emperors, but successfully concealed his vicious propensities so long as his father lived. The philosopher's wife Faustina was accused, perhaps unjustly, of gross immorality, but he never suspected her, and after her death took trouble about her deification. He persecuted the Christians, because they rejected the State religion, which he considered politically necessary. In all his actions he was conscientious, but in most he was unsuccessful. He is a pathetic figure: in a list of mundane desires to be resisted, the one that he finds most seductive is the wish to retire to a quiet country life. For this, the opportunity never came. Some of his Meditations are dated from the camp, on distant campaigns, the hardships of which eventually caused his death.
(emphasis my own)
There is also this translation:
"Let this always be plain to thee, that this piece of land is like any other; and that all things here are the same with things on top of a mountain, or on the sea-shore, or wherever thou choosest to be. For thou wilt find just what Plato says, Dwelling within the walls of a city as in a shepherd's fold on a mountain"
It might also be helpful to get a broad sense of Stoicism (see also here).
Perhaps in your translation the "empty field" is meant to invoke a Stoic notion that no matter what the human artifice may be upon it, wherever you may find yourself, you are still in that empty field?
Lastly, consider as well this earlier paragraph:
- Little of life remains to you. Live as on a mountain. For it makes no difference whether a man lives there or here, if he lives everywhere as a citizen of the world. Let men see, let them know a real man who lives according to nature. If they cannot endure him, let them kill him. for that is better than to live as they do.
I suppose these are easy enough sentiments for an Emperor to espouse... but one must also imagine Marcus Aurelius encamped in battle, making sense of the world and his place in it.
* Meditations, Marcus Aurelius, 1945 by Walter J. Black, Inc., translated from the Greek by George Long, revised and clarified by the Classics Club editors.