Hadrian's Wall was (for a while) the northern frontier of the Roman Empire. It was a stone / turf wall built around 122 AD, 120 km long, 3 meters wide and 5-6 meters high with towers along the way. Behind the wall were several forts among others that at Vindolanda.
The wall was attacked by the Caledonian's (Scots) over and over again and in 142 AD the Romans decided to build a wall to the north of it called Antonine Wall. This wall was smaller and built mostly in timber / turf. It was 63 km long, 3 meters high and 5 meters wide. It also had several forts behind it.
Around the year 410 AD, attacks on Rome / other parts of the Roman Empire by the Germanic tribes (Goths, Vandals, Cimbri, Teutons) led to Britain being abandoned in order to protect Rome. This left Britain in an unstable situation with no rulers. The Angels and Saxons of Denmark and northern Germany then moved in.