Imagine you are playing a game of tag with your friends. You don't want to get tagged, so you find a big tree and hide behind it. That tree protects you from being tagged.
In ancient Rome, people built walls around their cities to protect themselves from enemies. These were big walls made of stones and bricks that were very high and strong. The walls were like a big shield that protected the city from anyone who wanted to attack it.
Inside the walls, there were towers where people could watch and see if there were any enemies coming. They could also shoot arrows and throw rocks at the attackers to keep them away.
Sometimes, the walls had gates that could be opened to let friendly people in and out of the city. But those gates were very strong and could only be opened by the people who lived inside the city, so strangers couldn't sneak in.
The walls were an important part of ancient Roman society because they helped keep people safe. Just like the tree protected you from getting tagged, the walls protected the people in Rome from getting hurt or attacked.