I'm not sure what the laws are there, but I'm guessing this is next option is illegal there as it is here. You could hook the metal gates to an electric line. The screams would alert you.

The other way to do it is to use tripwire line or braided fishing line hooked up to an electronic alarm (pulls a pin or tab to alarm) or ammo blank alarm (pulls a trigger to fire the blank). Put it a few inches off the ground or attach it to a gate.

You can also use lasers as a tripwire device. They make them for driveways so it will make a doorbell-type noise to alert you that people are coming. This could be a great option if you have to come down a driveway or path to get to the horses, but you may get false alerts with deer. You can also use this between your outer and inner fence (if you have that style).