I heard that the city was favored due to the marshes nearby, which made sieging the city quite difficult. Kulikowsky seems to take this stand when he has the Goths go for Rome.
There are no extant accounts (that I know of) that give Ravenna credit for the supposed exceptional defensive qualities of the city. They are largely modern attributions.
It may have been those very marshes that made the city fall to invaders over and over again. The high water table and salination left the city bereft of natural sources of fresh water, food production and timber. When the only road leading to the city was blocked, all supplies had to be brought in by sea transport. Additionally, the court (and any armies present) would be trapped, leaving the besieger free to ravage the rest of the peninsula.
"[belisarius was] alarmed both for Rome and the whole Roman cause, since it was impossible to lend assistance from Ravenna in any case.... Indeed he repented having ever come to Ravenna at all...since by shutting himself up in that place he had given the enemy a free hand to determine the course of the war as they had wished." - Procop., BG VI 13.13-18
The Goths packed up and left for Rome because Alaric thought holding Rome hostage would be more effectual. Ioannes was defeated at Ravenna in 425; Avitus' patricius Remistus was killed at Classe in 456; Nepos took the city by force in 474 and was in turn evicted by Orestes; Orestes' brother Paulus was defeated at Ravenna by Odoacer, who was starved into submission by Theodoric in 493. Belisarius succesfully besieged the city in 540 and abandoned it when besieged in 545. The Lombards, finally, ravaged the city repeatedly in the seventh century, after which it would sink into oblivion.
Most of this information comes from Andrew Gillett's 'Rome, Ravenna and the Last Western Emperors', Papers of the British School at Rome, vol.LXIX (2001), pp. 131-167.