Yet another vote for Castelnaud...but if you can do both, I’d start at Beynac.
Beynac was partially under construction when we were there last September so some areas were closed, but there were large areas open & it was fun peeking in at the reconstruction team meeting in one of the rooms, discussing what they were doing next. Great views and a little area with food, gelato, etc. just outside.
Castelnaud was magnificent & had a good tour in English showing off the trebuchet & other “weapons of war” as the guide explained the 100 Years War. Again, great views and food / souvenirs available outside. Be wary of GPS guidance to Castelnaud...ours tried to take us down a pedestrian path! Maybe if you enter “Castelnaud parking” you’ll have better luck...or maybe follow the signs along the road?
It is a beautiful area...enjoy it!