Castell Pen-yr-Allt is the site of a Medieval castle, but there’s a strong possibility that the hill was also used during the Iron Age as a fort. I’ve not visited the it, so I’ve no idea what’s visible on the ground.
Yesterday I had a look through the Cadw records for Castell Pen-yr-Allt, held at the National Monuments Record of Wales. In the paperwork I looked at there was no reference to an Iron Age settlement on the site of the castle. It looks like Cadw have yet to be pursuaded of a prehistoric connection.
That’s not to say that there wasn’t a prehistoric settlement though. My research was incredibly rushed and superficial (I visited during my lunch hour) so there’s plenty of other material to be sifted through. Just a quick look at the catalogue shows that Cambria Archaeology and the RCAHMW both associate the site with the Iron Age.