Opening onto the plaza is the oldest theatre in Spain. The 17th-century Corral de Comedias is an evocative tribute to the golden age of Spanish theatre, with rows of wooden balconies facing the original stage, complete with dressing rooms. Once daily (twice on Saturday) visits become 'theatrised' with costumed actors replacing the audio guide: this costs €3 more. It's still used for performances on Saturday evenings from the end of March to mid-December (though not in July); buy tickets via the website.
The history of the theatre is intriguing. It was founded in 1628 by a wealthy priest, but, after a century of performances, was closed during the cultural clampdowns of King Felipe V. After that, the theatre was pretty much forgotten until a local inn-owner found a deck of old playing cards in the 1950s. Subsequent excavations on the site in Plaza Mayor led to the rediscovery of the theatre, which reopened for performances in 1954.