Stirling Castle once was a Royal residence of the kings of Scotland. Being situated near the border to England, the castle was of strategic importance for centuries before it became the seat of kings. When the Scots started to convert it into a Royal residence, the builders included a curious code into the ceiling of the king’s bedchamber. The code was cracked to reveal the music hidden within.
Looking for a good pub quiz question? This one might catch some people out. Europe has a surprising number of monarchies, ranging from the well known to the obscure. They cover the spectrum from constitutional monarch through power sharing to absolute ruler. The youngest monarchy was instituted in 1929 (and that's a clue).