The Illyrian frontier was covered by a line of castles and fortresses; and though the greatest part of them consisted only of a single tower, with a small garrison, they were commonly sufficient to repel, or to intercept, the inroads of an enemy, who was ignorant of the art and impatient of the delay of a regular siege. But these slight obstacles were instantly swept away by the inundation of the Huns. They destroyed, with fire and sword, the populous cities of Sirmium and Singidunum, of Ratiaria and Marcianopolis, of Naissus and Sardica; where every circumstance of the discipline of the people and the construction of the buildings had been gradually adapted to the sole purpose of defence. The whole breadth of Europe, as it extends above five hundred miles from the Euxine to the Adriatic, was at once invaded, and occupied, and desolated, by the myriads of barbarians whom Attila led into the field. The public danger and distress could not, however, provoke Theodosius to interrupt his amusements and devotion, or to appear in person at the head of the Roman legions. But the troops which had been sent against Genseric were hastily recalled from Sicily, the garrisons on the side of Persia were exhausted; and a military force was collected in Europe, formidable by their arms and numbers, if the generals had understood the science of command and their soldiers the duty of obedience. The armies of the Eastern Empire were vanquished in three successive engagements; and the progress of Attila may be traced by the fields of battle. The two former, on the banks of the Utus and under the walls of Marcianopolis, were fought in the extensive plains between the Danube and Mount Hæmus. [In the latter battle Arnegisclus, the Roman commander, was slain.] [Sidenote: [447-448 A.D.]]