Caerlaverock Castle
A perfect ruin!

When the glow of the afternoon sun warms the standstone of Caerlaverock Castle, it's hard to imagine a more impressive fortification.
It's filled moat and imposing gatehouse are particularly fine – and all the more so considering they're of 13th-century vintage.
Edward I laid the castle to siege in 1300, and it was seriously battered again in 1312 (though by Scots rather than the English), and again in 1356-7.
So it's fair to say there's been a lot of rebuilding over time, but there have also been various improvements based on fashion: in the 1630s the incumbent Earl of Nithsdale its interior received an ornate Renaissance facade, which included all sorts of mythological scenes.
But within six years of the work's completion a thirteen-week siege by the Covenanters forced Nithsdale's surrender and made him watch as the castle was reduced to its present state. No-one has lived in it since.