concentric, adj. – having a common center

“The great city, fortified by three concentric round walls was the administrative center of Islam and by the tenth century had a population of about 1.5 million people.” ( The west in the world)

I was confused about the meaning concentric. I knew it must have had something to do with a central point. I just wanted to make sure what the meaning was. But now I know that the great city was 3 curved walls centered around the city.