Nothing now but the river Tigris intervened between Julian and the great city of Ctesiphon, which was plainly the special object of the expedition. Ctesiphon, indeed, was not to Persia what it had been to Parthia; but still it might fairly be looked upon as a prize of considerable importance. Of Parthia it had been the main, in later times perhaps the sole, capital; to Persia it was a secondary rather than a primary city, the ordinary residence of the court being Istakr, or Persepolis. Still the Persian kings seem occasionally to have resided at Ctesiphon; and among the secondary cities of the empire it undoubtedly held a high rank. In the neighborhood were various royal hunting-seats, surrounded by shady gardens, and adorned with paintings or bas-reliefs; while near them were parks or "paradises," containing the game kept for the prince's sport, which included lions, wild boars, and bears of remarkable fierceness. As Julian advanced, these pleasaunces fell, one after another, into his hands, and were delivered over to the rude soldiery, who trampled the flowers and shrubs under foot, destroyed the wild beasts, and burned the residences. No serious resistance was as yet made by any Persian force to the progress of the Romans, who pressed steadily forward, occasionally losing a few men or a few baggage animals, but drawing daily nearer to the great city, and on their way spreading ruin and desolation over a most fertile district, from which they drew abundant supplies as they passed through it, while they left it behind them blackened, wasted, and almost without inhabitant. The Persians seem to have had orders not to make, as yet, any firm stand. One of the sons of Sapor was now at their head, but no change of tactics occurred. As Julian drew near, this prince indeed quitted the shelter of Ctesiphon, and made a reconnaissance in force; but when he fell in with the Roman advanced guard under Victor, and saw its strength, he declined an engagement, and retired without coming to blows.
Julian had now reached the western suburb of Ctesiphon, which had lost its old name of Seleucia and was known as Coche. The capture of this place would, perhaps, not have been difficult; but, as the broad and deep stream of the Tigris flowed between it and the main town, little would have been gained by the occupation. Julian felt that, to attack Ctesiphon with success, he must, like Trajan and Severus, transport his army to the left bank of the Tigris, and deliver his assault upon the defences that lay beyond that river. For the safe transport of his army he trusted to his fleet, which he had therefore caused to enter the Nahr-Malcha, and to accompany his troops thus far. But at Coche he found that the Nahr-Malcha, instead of joining the Tigris, as he had expected, above Ctesiphon, ran into it at some distance below. To have pursued this line with both fleet and army would have carried him too far into the enemy's country, have endangered his communications, and especially have cut him off from the Armenian army under Procopius and Sebastian, with which he was at this time looking to effect a junction. To have sent the fleet into the Tigris below Coche, while the army occupied the right bank of the river above it, would, in the first place, have separated the two, and would further have been useless, unless the fleet could force its way against the strong current through the whole length of the hostile city. In this difficulty Julian's book-knowledge was found of service. He had studied with care the campaigns of his predecessors in these regions, and recollected that one of them at any rate had made a cutting from the Nahr-Malcha, by which he had brought his fleet into the Tigris above Ctesiphon. If this work could be discovered, it might, he thought, in all probability be restored. Some of the country people were therefore seized, and, inquiry being made of them, the line of the canal was pointed out, and the place shown at which it had been derived from the Nahr-Malcha. Here the Persians had erected a strong dam, with sluices, by means of which a portion of the water could occasionally be turned into the Roman cutting. Julian had the cutting cleared out, and the dam torn down; whereupon the main portion of the stream rushed at once into the old channel, which rapidly filled, and was found to be navigable by the Roman vessels. The fleet was thus brought into the Tigris above Coche; and the army advancing with it encamped upon the right bank of the river.