I have just added this to the map of places to fly your drone at Drone Scene:
Land owner permission requirements unknown.
Bronllys castle is, as CADW term is, a"sturdy stone tower with a turbulent history".
In truth there isn;'t much to it, but if you brave the 200 or so steps all the way to the top you'll be rewarded with a magnificent view across to Pen y fan in one direction, and bits of Herefordshire in the other. I guess it depends on how much you like old fortifications.
CADW is a bit like the NT, and they aren't fans of drones but as there is no one around to ask or stop you (except at close/open times) then rock on I say. For parking there is a lay-by directly opposite and then it is a very short walk to the steps… and there's a few.
CADW page is: Bronllys Castle | Cadw
The originator declared that this location was not inside a Flight Restriction Zone at the time of being flown on 01/06/2023. It remains the responsibility of any pilot to check for any changes before flying at the same location.