Yes, i know this is the England forum but I don’t think that you all read the Welsh forum section as well - where I have also posted this:>
I see that Transport for Wales (Welsh Train Company) have a 2 for 1 deal for entry to CADW Castles. (CADW means castles under the control of the Welsh Government). So, be sure to retain your old ticket as proof of travel by train. Note that if you put it into a ticket barrier slot that you won’t see it again so see a member of staff instead and they will open the gate.
https://tfw.wales/ways-to-travel/rail/savings-and-offers/cadw
Thanks, James! That's good to know. And FWIW, I read everything in the All Topics, so it doesn't really matter where it's posted.