St. Donat's Castle (Atlantic College) as The Monastery
Some outside areas of the Monastery as well as the main living room where the meetings between the regular crew and their doppelgängers.
Source: The Doctor Who Locations Guide
“The Rebel Flesh” Filming Locations
Doctor Who episode “The Rebel Flesh” was filmed in Cardiff in the United Kingdom.
Neath Abbey as The Monastery
The doppelgänger chamber where the Monastery crew creates their 'gangers' from the flesh.
Source: The Doctor Who Locations Guide
Caerphilly Castle as The Monastery
The TARDIS lands on the shore outside of the Monastery.
Source: The Doctor Who Locations Guide