Bounties attract serious brainpower to the challenge.
This idea simply aims to provide a way in which to secure your home without having to enclose your yard with a massive security wall or fence.
In South Africa, due to the high crime rate, almost all yards are now enclosed with massive security structures. While it is obvious that safety will always be a priority over aesthetics, I can imagine that it would be best to be able to enjoy both.
The idea is to simply have a system installed that allows you to have your wall/fence erected with the push of a button, or as soon as your alarm goes off.
This could be done in one of three ways (that I can think of).
You can have a low wall that contains the remaining wall or fence, which would then be raised by a hydraulic system when needed. (This might be the easiest option)
The entire wall/fence can be stored underground, also to be raised by hydraulics. This would be similar to hydraulic road blockers, with the main difference being that it would need to be much taller.
3. The wall could be horizontal, parallel to the ground when not in use, then be lifted into an upright position when needed. (This would require a lot of open space on the ground)
This system could then be activated at night, whenever you feel unsafe, or when you are away. It should also be connected to your alarm system, effectively trapping any trespassers inside when the alarm is activated.
Obviously a brick wall would be the sturdiest option, but it might require very strong hydraulics to move. An alternative would be a very tall fence, which won't weigh much, but could then also be penetrated more easily.
This allows homeowners the option of privacy and security when they want, but also the option to have an open yard with a view of the neighborhood or whatever view they might have.