I would advise only ever visiting with your drone "before or after hours". There is ample parking on the road outside and then you can access the site via the footpath.
I say this, and also mention that permission is unknown as it is technically CADW managed , and as such, they state they don't allow drone flights over or on/from their land. However, the bit of land that is the Taff Trail is queryable. This footpath joins onto the Taff Trail so is always open 24x7.
As ever, I'm only pointing out what I have done and where I have flown - the choice on when or if you do is yours.
Land owner permission requirements unknown.
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Co-ordinates: 51.53589, -3.254681 • what3words: ///large.turkey.front