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.

See also J.N.D. Kelly, The Oxford Dictionary of Popes (Oxford: Oxford University Press, 1986; rpt. 1989), p. 104:

Leo's immediate task was the defence of Rome against the Saracens and the repair of the damage inflicted by them in 846. With extraordinary energy he strengthened the city walls and, reviving plans of Leo III, constructed new walls, with financial help from Emperor Lothair I (840-55), on the right bank of the Tiber, bringing St Peter's, hitherto exposed to enemy attack, within the defensive system and creating the 'Leonine city'. These new defences were solemnly dedicated on 27 June 852. In 849 he organized the fleets of Naples, Amalfi, and Gaeta and defeated the Muslims in a decisive sea-battle just outside Ostia. In 854 he rebuilt Centumcellae, destroyed by them, on a more secure site, naming it Leopolis (today Civitavecchia), while at Porto he settled Corsican refugees as a defensive garrison.

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.

This map shows the location of the Leonine wall (west, left): Some of the Leonine wall still remains — see Sheila Gibson and Bryan Ward-Perkins, "The Surviving Remains of the Leonine Wall," Papers of the British School at Rome 47 (1979) 30-57 and 51 (1983) 222-239.