Fill the moat with loads of piranhas?
Hmm - not keen on that form of defence.
Pirahnas are indeed carrion scavengers, but I'm not certain how "gamey" they like their carrion - particularly ripe cadavers might not be terribly appetising.
It is unfortunate that the "James Bond Villain" species of Pirahna isn't really terribly accurate - a boiling mass of fishies stripping a man to the bone in seconds is a bit of a Hollywoodism. Assuming your cadaver is alone and palatable he might get a few bites, but will doubtless emerge on the other side of your moat intact enough to be a threat.
Plus, you'd need an awful lot of hungry pirahnas to ensure they didn't get bored after meal No. 1.
Additionaly, one good winter here in northern Europe would probably be a bad thing for our tropical chums.
Can we shoot the welsh if they stray onto the village green, sire?
Just you try it....
Precisely.
I believe that's only allowed on a market day after sunset.