The Roman Empire was vast and extensive, covering almost all of Europe. Britain also came under its rule for 400 years. First Julius Caesar visited in 55 BC, but he did not stay. Eventually, in 43 AD permanent conquest of the British Isles began and Britain became a Roman province. At the time, Britain was home to many tribes, some of whom had been already trading with the Romans and were happy to welcome the invaders. Others however, like the Picts of the north, in what is now Scotland, were particularly unimpressed by the Romans and kept rebelling against the conquerors, sometimes even venturing south to attack.
Rome got a new emperor from 117 to 138 AD. His name was Hadrian and he visited Britain, which was one of the farthest outposts of his empire, in 122 AD. It was Hadrian who decided to build a wall between England and Scotland to defend his territories in England.