In fact, those walls remained unbreached until the Ottomans came along in 1453. They were constructed when little Theo was only about 12 or 13 and was still under the care of his older sister (Pulcheria) and the Praetorian prefect Anthemius, so I guess they could just as well been called the Pulcherianthemian Walls (or the Arcadian Walls, since it was he who supposedly got the whole project underway).
In 423, when Honorius (emperor of the West) died of swelling, Theo II was thinking of going old school and running both halves of the empire on his own. As he pondered his decision, a civil servant named Johannes stepped up and took the western throne for himself.
Then Theo II's man Valentinian III (a six-year-old) was installed as emperor...
VALENTINIAN III 425-455
It was around this time that Theo II began paying the Huns large sums of gold to keep them away. In 424, the Huns accepted 350 pounds of gold. In 433, Attila convinced Rome to double that amount and collected 700 pounds of gold. In 443, after the Huns defeated a couple of Roman armies, they demanded 2,100 pounds of gold. And in 450, Theodosius II fell off of his horse and died.
Meanwhile, out west, Valentinian III's general Aetius had teamed up with a Visigothic king and they led an army out to face the Huns in 451. At the Catalaunian Fields, they managed to do something no one had previously been able to do: they stopped the Huns.
I mean, the Huns were still able to ransack much of Gaul, and the Roman/Visigothic army was reduced to life support, but a stop is a stop.
However, Valentinian III, a hedonistic ruler who fancied sorcery and astrology and wading face-first into Christian controversies (in 445 he passed a law that recognized the primacy of whichever bishop happened to be in charge of Rome) felt threatened by the success of Aetius. One day, when Aetius showed up in court to present a report, Val III accused him of drunkeness, rushed him all at once, and gave him a terminal sword stroke to the side of the head.
Two years later, two of Aetius' supporters responded in kind and killed Valentinian III.
If you look a bit closer, you'll see that the man who incited Valentinian III to kill Aetius is the same man who later persuaded the two Aetian supporters to kill Valentinian III; the wealthy senator Petronius Maximus.
There's a murky story about how perhaps the lusty Val III seduced Pet Max's wife and Pet Max had been plotting his revenge ever since. Whatever the backstory, after Val III's assassination, Petronius found himself in charge of Rome... tentatively. The thing is, he didn't have much in the way of support, so when the Vandals rode on Rome in 455, his people abandoned him. He tried to sneak out of the city but, as soon as he got outside of the city walls, an angry mob spotted him and stoned him to death. Then they mutilated his corpse and threw it in the Tiber.
When the Vandals showed up, they spent two full weeks sacking the city, but out of respect for Pope Leo, they agreed to refrain from any excessive arson or murder. On their way out, they took the empress (formerly married to Valentinian III and then Petronius Maximus) and her daughter.
Over in Gaul, the Visigothic king Theodoric II saw an opportunity to install his own man on the Roman throne and promoted his pal Avitus.
When Avitus arrived in Rome (from Gaul) to officially take his seat, a powerful Romanized Germanic commander named Ricimer (stationed at Ravenna) promptly rebelled. Ricimer rode out against Avitus and defeated him, but decided to spare his life. Instead of killing Avitus, Ricimer made him become a bishop, which effectively took him out of the game (of thrones).
Ricimer's man was a senator/general named Majorian.
Majorian managed to reconquer parts of Spain and Gaul after strengthening his forces with barbarian mercenaries.
...but Ricimer got nervous about Majorian's growing power and decided the best thing to do would be to arrest him and torture him and cut off his head.
Ricimer replaced Majorian with a puppet named Severus III.
At this point, Ricimer needed eastern military aid but the east would only help him under the condition that they be allowed to choose the next western emperor. They chose Anthemius.
Anthemius was a general who'd proven himself capable in the past, fighting successfully against Goths and Huns. When he rode against Vandals and Visigoths in 467 and 470, however, success was elusive. Ricimer, not a fan of Anthemius in the first place, rode on Rome in 472 and beheaded Anthemius in St Peter's Basilica.
Having done away with Anthemius, Ricimer installed another Roman noble. This one was named Olybrius.
Olybrius had all the makings of a willing puppet but Ricimer died in 472 and Olybrius quickly followed in kind, succumbing to dropsy only a few months later.
Ricimer's nephew Gundobad took over as puppet master and installed the Imperial Commander Glycerius as his Pinocchio.
The eastern empire did not recognize Glycerius and sent out their own man, Julius Nepos, to take over.
Julius Nepos, former governor of Dalmatia, deposed Glycerius without a fight and sent him to Dalmatia to be a bishop.
But Julius Nepos' own commander, Orestes, deposed him and forced him to flee to Dalmatia where he is thought to have been killed by Glycerius and/or his own men around the year 480.
Orestes installed his own son, the appropriately named Romulus Augustulus, as emperor in 475.
Both Orestes and Romulus Augustulus were dominated by Odoacer and the Germanic mercenaries who eventually demanded Italian land for themselves. Orestes balked, so they killed him and deposed Romulus, allowing the boy to retire to Naples with a modest pension. He was so unimportant that no one took the trouble to make note of how or when he died.
And that's it. That was the end of the Roman Empire and the men who sat on the throne as rulers. It was a pretty gradual thing and nobody living at the time made any real fuss about Romulus Augustulus. The empire had been catatonic for years and it ended not with a scream but with a whisper, a whisper so quiet that it wasn't even noticed.
Kind of anticlimactic, no? The eastern empire kept on going (we call it the Byzantine Empire) until 1453 when the Ottoman's took over, but the western chunk of the Roman Empire was done. Some people believe it stopped being the Roman Empire long before 476. Some say it ended with the death of Commodus or during the Third Century Crisis or when Constantine came to power or when Rome itself was sacked... the point is, for me, that it ended and that, in its wake, it left a fascinating legacy.