Agger was a fancy word used by the ancient Romans to describe a big wall made out of dirt and stones! The wall was built to protect their cities and towns from enemies who might want to attack them.
When building the agger, the Romans would pile up dirt and rocks to create a really strong wall. They made the wall super thick so that it could stop people from trying to walk or drive through it.
But that's not all! The agger was really important because it helped the Romans move around their cities and towns more easily. The wall could be used like a big road or path, and it helped people to travel quickly and safely.
So basically, the agger was a big and strong wall that protected the Romans from danger, and helped them move around their cities and towns!