With My mum being an old school gamekeeper
Her security was getting an old tractor drag and tipping it spike side up and leaving in a place where, as kids, we were nbanned from and as a thief, would prob be one of the ways you would come in or go out.
She also used to leave a lot of sharp stakes at ball height
But by far the best deterrent are, as said above, the air mines with trip wires.
Basically its just a blank cartridge with a fire pin and trip wire
These were customised so they would have rock salt and dried peas in the cartridges in stead of blanks.
They were pointed the opposite way round and instead of blowing down to the floor, blow straight up between your legs.
There were lots of times they went off but never found anybody there!!
Ended up stopping anything getting nicked from the farm for years and years.
But also made you very cagey about where you were allowed to go and where not.....