The principal canal of Augustus poured a copious stream of the waters of the Po through the midst of the city, to the entrance of the harbor; the same waters were introduced into the profound ditches that encompassed the walls; they were distributed by a thousand subordinate canals, into every part of the city, which they divided into a variety of small islands; the communication was maintained only by the use of boats and bridges; and the houses of Ravenna, whose appearance may be compared to that of Venice, were raised on the foundation of wooden piles.
The adjacent country, to the distance of many miles, was a deep and impassable morass; and the artificial causeway, which connected Ravenna with the continent, might be easily guarded or destroyed, on the approach of a hostile army These morasses were interspersed, however, with vineyards: and though the soil was exhausted by four or five crops, the town enjoyed a more plentiful supply of wine than of fresh water.
[61] The air, instead of receiving the sickly, and almost pestilential, exhalations of low and marshy grounds, was distinguished, like the neighborhood of Alexandria, as uncommonly pure and salubrious; and this singular advantage was ascribed to the regular tides of the Adriatic, which swept the canals, interrupted the unwholesome stagnation of the waters, and floated, every day, the vessels of the adjacent country into the heart of Ravenna.

The gradual retreat of the sea has left the modern city at the distance of four miles from the Adriatic; and as early as the fifth or sixth century of the Christian aera, the port of Augustus was converted into pleasant orchards; and a lonely grove of pines covered the ground where the Roman fleet once rode at anchor.

[62] Even this alteration contributed to increase the natural strength of the place, and the shallowness of the water was a sufficient barrier against the large ships of the enemy.

This advantageous situation was fortified by art and labor; and in the twentieth year of his age, the emperor of the West, anxious only for his personal safety, retired to the perpetual confinement of the walls and morasses of Ravenna.

The example of Honorius was imitated by his feeble successors, the Gothic kings, and afterwards the Exarchs, who occupied the throne and palace of the emperors; and till the middle of the eight century, Ravenna was considered as the seat of government, and the capital of Italy.