Beaumaris Castle overlooks the Menai Strait on Anglesey in north Wales and was built by King Edward I in 1295 as part of his Iron Ring of Castles, a series of castles built around Gwynedd to control the Welsh. Beaumaris itself was begun in response to a rebellion led by Madog ap Llywelyn.
In order to build Beaumaris, Edward destroyed a Welsh llys (palace), along with the entire Welsh town of Llanfaes, which was the most important trading port in Gwynedd at the time. The people were moved inland to Newburgh, and English settlers were brought in to populate Beaumaris. The English crown spent a total of 15,000 pounds on the castle, but it was never finished, the work finally being abandoned in 1330.
Key features to visit within the castle are the many passages within the walls, the numerous latrines, and the moat that encircles it.
Although the castle itself was built after the time periods of any of my books, David and William time travel to an adjacent field after an assassination attempt in Champions of Time.