The mighty Roman Empire was ruled from within the walled city of Rome. It was a brick and stone wall standing 33 feet tall with guarded entrances, ramparts, and other defenses built into it. The gates could even be sealed in the event of an attack.
However, in 470-ish CE, the first of many tribal invasions of Rome began. Germanic tribes from the interior of Europe began to attack Rome in 468 CE. The first ones were not terribly successful and were defeated by the walls.
This luck would run in 475(?) CE when the Vandals would come to Rome. The Vandals had battled The Huns and the Visigoths on their way to Rome and defeated them all. Roman scouts even warned Rome that the Vandals were heading their way. Yet, Rome seemed unprepared when the Vandals showed up at the entrance gates of Rome.
In many ways the Vandals were similar to the Picts. They thrived on war and were very adept at changing military strategies to deal with different situations. Including the Walls of Rome. The Vandals decided not to attack the walls directly, they went through them. They attacked a gate and before the gate could be sealed, entered Rome.
And in a huge “oops moment” the gates were sealed after several hundred Vandals entered Rome. It turned out having Vandals on the inside and the outside was a problem for Rome and a good thing for the Vandals. The Vandals outside kept the guards on the ramparts busy while those on the inside took control of the gates. Eventually the Vandals outside were let in and the sacking of Rome left the city with so much damage that soldiers that guarded the walls had to be moved to repair, paving way for other tribes to enter Rome.
The Vandals’ attack on Rome was so well done, they expanded from the interior of Europe to set up a kingdom in Northern Africa (territory owned by Rome at the time) that they ruled for about 100 years. And yes, the modern word “vandalism” comes from the Vandals sacking of Rome.