Beaumaris Castle has been described as an 'unfinished masterpiece' and the 'greatest castle never built'. Along with Caernarfon, Conwy and Harlech, it is one of four castles from the reign of Edward I designated UNESCO World Heritage Sites. They formed part of his 'iron ring' of castles in North Wales, built to enforce his authority over the defeated Welsh.

All were designed and built by Master James of St George, the greatest military architect of the age. With Beaumaris he was able to start with a black canvas, rather than having to develop plans from the remains of an existing fortification. What he came up with was a fortress built on flat, marshy land, with four concentric lines of defensive walls within walls that included a sea-filled moat with its own dock and access to the open sea. 

The castle was never fully completed due to a lack of finances and the King's attention turning towards Scotland.