The last castle in Edward I's Welsh building programme, Beaumaris was also to have been the largest, with £15,000, up to 3,500 workmen and 35 years invested in the project before the King reallocated resources to Scotland and the first draft of 'Braveheart'.

Though unfinished and hence less immediately spectacular than Conwy or Caernarfon, Beaumaris is often considered the finest castle in Edward's 'iron ring' of North Wales and is, unsurprisingly, a designated World Heritage site. Designed by Master James of St George in 1295, it's an ambitious example of technically-perfect military architecture, its symmetrical layout providing four concentric layers of defence, beginning with the 5.5 m wide moat. The towers never reached their intended height, but they're still impressive and I quite like their squat appearence.
Part of the reason for the idealised layout was that the castle was constructed on previously-unused low, flat land (hence the name 'fair marsh', 'beau mareys' in Norman French), so Master James was not constrained by terrain or existing fortifications. The coastal location offered a further advantage in that a tidal dock could be integrated into the moat, enabling fully-laden supply ships to directly approach the main gate.

It's odd to think that the castle saw no significant action until the English Civil War in the 17th Century, by which time its once-revolutionary design was approaching obsolescence: vertical walls were no longer the optimum defence in an age of artillery.