If I ever build a castle, I'll have it surrounded by a large moat with enormous Central Asian shepherd dogs patrolling the perimeter in order to protect me from the enemies.
Naturally.The Kronborg Castle in Helsingør, Denmark, on the other hand, seems to have only needed one guard: instilling fear into all those around was...a white swan. Granted, this particular swan was in the perpetual fishing mode, so who knows what kind of horrifying monsters--that came by the way of the Baltic--it managed to capture below the water surface.
More important, Kronborg has a bit of extra protection. After all, Holger Danske is asleep below ground till he is needed again.
Just in case.