Segontium Roman Fort, Gwynedd

I didn’t quite manage to see this, but it’s in the outskirts of Caernarfon, on the A4085 Beddgelert road. When I got there, the main part was locked up. There’s also a museum building, (not NT). The seasonal opening hours were Tue-Sun 12.30 to 16.30. Another part of the site is across the road, and IIRC can be accessed anytime.
Looks like it would be worth a look if you are in Caernarfon. You can park in the road.