Doesn't have such a negative visual aspect as a perfectly secure fence, doesn't involved major works such as a "medieval" moat etc? Would look like theres nothing there.
Fairly cheap as well.
Or turn over the outer lawns to rabid badgers.
Mantraps would also work. I.e. trap doors that open up with a 20ft fall (you could put a net at the bottom if you wanted to be nice.
Basically, it would be rather simple to create an invisible moat and anyone that gets past the invisible moat should probably just
be shot as it's then obviously not an accident.