Honey08
Waffled a lot!
I'd fence it as you said, with the posts and elec wire/rope, put a locked gate on so that it can be used when you need it, then get some big bales stacked right behind the gate and any places you could lead a horse through, so that even if the break the gate they can't get in.
If that doesn't work, then I would plough..
If that doesn't work, then I would plough..