How was the city of rome sacked in 410 AD?

Literally from vulnerability,

In the years of glory, the Roman Empire protected its capital by stopping invaders in Northern or SOuthern Italy before they got to Rome. Rome is also pretty far from the sea, which was well guarded too so it wasnt easy.

In 370AD though, Constantine moved the capital of the Roman empire from Rome to Constantinople (Istanbul). With the capital went the emperor, the legionares, the senate, and all of the body guards and protective solders who went with them.

Years after, the city was weak and vulnerable, most of the residents still lived in Rome and many merchants as well, like gold, silk, ivory, and spice merchants. Barabarian hords from the Germanic States to the north saw this as a golden opprotunity, the broke into the city and ransacked it. Romans got together and offered them a bribe to leave the town and they did, but the roman army had failed and the western provinces had fallen.