If you go to Córdoba you will see that the mosque is very large, and the Catholic Cathedral was built in the middle of it. The rest of the mosque is still there around the inserted cathedral.
The whole thing is well worth a visit if you are in Spain.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.