Complex sentence: The museum has an extensive collection found at the site of Sala-Chellah, a Mauretanian and Roman town which flourished up to 4th century and became a royal necropolis in the 13th century under the Merinids.
Split into simpler sentences: The museum has an extensive collection found at the site of Sala-Chellah. This was a Mauretanian and Roman town, which flourished up to the 4th century and became a royal necropolis in the 13th century under the Merinids.