Laudator Temporis Acti

John M'Clintock and James Strong, Cyclopaedia of Biblical, Theological, and Ecclesiastical Literature, Vol. V (New York: Harper & Brothers, Publishers, 1891), p. 360:

Leo IV, Pope, was a native of Rome, and succeeded Sergius II in 847. He was hastily elected, and consecrated without waiting for the consent of the emperor Lotharius, because Rome was then threatened by the Saracens, who occupied part of the duchy of Benevento, and who a short time before had landed on the banks of the Tiber, and plundered the basilica of St. Peter's on the Vatican, which was outside of the walls. Leo's consecration, however, was undertaken with the express reservation of the emperor's rights, and when, in order to prevent a recurrence of the violence of the Saracens, Leo undertook to surround the basilica and the suburb about it with walls, the emperor sent money to assist in the work. The building of this Roman suburb occupied four years, and it was named after its founder, Civitas Leonina. Leo also restored the town of Porta, on the Tiber, near its mouth, settling there some thousands of Corsicans, who had run away from their country on account of the Saracens. Towers were built on both banks of the river, and iron chains drawn across to prevent the vessels of the Saracens from ascending to Rome. The port and town of Centum Cellae being forsaken on account of the Saracens, Leo built a new town on the coast, about twelve miles distant from the other, which was called Leopolis; but no traces of it remain now, as the modern Civita Vecchia is built on or near the site of old Centum Cellae.

Even Gibbon praised him — see Edward Gibbon (1737-1794), The History of the Decline and Fall of the Roman Empire, ed. H.H. Milman, Vol. V (New York: Harper & Brothers, Publishers, 1900), p. 315:

[T]he unanimous choice of Pope Leo the Fourth was the safety of the church and city. This pontiff was born a Roman; the courage of the first ages of the republic glowed in his breast; and, amidst the ruins of his country, he stood erect, like one of the firm and lofty columns that rear their heads above the fragments of the Roman forum.

Some of the Leonine wall still remains.