Defence
Everybody knows about the Great Wall of China, but how many people in know about England's wall? It cannot really be described as 'great', but it has an interesting history. For a start, like the wall on the mainland, it was built as a defence to keep the 'barbarians' out.
In China's case, the 'barbarians' were tribes on the Mongolian steppes. In England, they were the Picts. The Picts lived in what is now Scotland. The Romans invaded England in the year AD 45. In the year 122, the Roman Emperor Hadrian visited England, and learned that the Picts were making it difficult to rule northern England. He decided a wall was the answer.
From wood to stone
The Romans started building the wall the same year the emperor visited. Historians think it took them about 20 weeks to build the original 117-kilometre-long wall. They did it so quickly because it was built from wood.
It took the Romans six more years to turn it into a stone wall. Every Roman mile, there was a lookout post for soldiers. A Roman mile was 1,000 paces - it is about 1.5km. They also built much larger forts at different places along the wall. Nobody knows the exact number, but maybe as many as 17. Lining the far side of the wall was a 3-metre deep ditch of water - like a moat. There were about 10,000 soldiers defending the wall.