This roman theatre was well preserved underground and discovered in 1951 during some building repairs. It dates back to the first century and was used until the 3rd century. When the Moors took over the area they reused many of the columns and other building material in building the alcazaba on the hill above the theatre.
There is a great short movie about the theatre, and then an interactive room where at the touch of a screen/table, you can gain more information on the historical aspects and how the theatre was uncovered, etc. There is no charge for this site. Definitely worth spending some time here. You can then walk on the 'boardwalk' that surrounds the theatre and see it all up-close-and-personal.