This review is going to be a little different from my usual movie reviews.
The Purge is a movie about an alternate version of the United States where one night a year they have an annual "purge" where violent people can purge themselves of their desire to hurt other people, and the poor and homeless can be eliminated from public concern; killing homeless people is encouraged. The main character, played by Ethan Hawke, is a salesman for a home security company whose home come under assault by a group of evildoers.
The issue I have with this security system that he installed, and that's what I'm going to be reviewing here. The system was described by Ethan as being 99% effective in testing, but it was not designed for worst case scenario. He casually mentioned that someone could tunnel underneath to get to them. Clearly, it didn't take that much. Let's look at his controls one by one.
Cameras: Cameras are a detective control and a deterrent. In this case, they are only a detective control, as there is no actionable crime that can be committed during The Purge (aside from very limited protections surrounding members of the government). There is no negative consequence for being videotaped doing bad things, so there is no deterrent. So, these are now only a detective control - you will know when someone bad is coming and what direction they are coming from (assuming they don't target the cameras with rifles or something). The fact is, cameras and the monitors that comprise most of this security system don't actually stop anyone from doing anything.
Incidentally, there are several views of the outside of the house that appear to be coming from locations outside the property line. It's unclear what the connections are to these cameras or how the legal right to place them on other people's property was established or (most importantly) what guarantee of the integrity of the feeds there is.
All of these cameras are to inform the person watching the feeds (which this family seemed to rarely do) so that they might react in some way to the outside. From the evidence in the film, they are given no procedures for dealing with anything they might encounter, and they only have microphones on the outside - no speakers to interact with anyone.
Bars: Bars on the doors are a reasonable precaution against casual break-ins. Anyone with a crowbar, saw, or hammer can get through them with enough time, and having glass that is not bullet proof means that this is still a prime candidate for entry. Still, this makes a good single layer of defense with a preventative control if only all of the windows had had bars as well. We can clearly see in many shots that not all of the windows have bars.
Magnetic Locks: Supplementing the bars are electromagnetic locks. These are good - we like locks for security. There are questions about the requirement for continuous power to keep them on or if they might be solid state electromagnets that might default to lock, but we have to assume these are going to default to secure. However, this puts us in the area where we have to talk about the control center in front of all the monitors. For some reason, no one mans them during The Purge, and the young son has been given the code that deactivates all of the locks. There is no justification for giving this kid the power to disarm your entire house - this is especially odd as Ethan demonstrates that there is no code required to re-enable the locks moments later.
Guns: This is not technically part of the security system, but there is a gun cabinet with keycode. They appear to have several small arms, but they only get one revolver out at the start of The Purge. Eventually, they raid it for more, and Ethan grabs a shotgun with a chainsaw grip mod. This is not the best weapon for defense against multiple home invaders, and the chainsaw mod seems to actually slow down the action more than speed it up.
Power: You pretty much have to assume that you will be cut off from the grid at some point in the night. They had backup power, but it was under-scaled. When the power was cut, there was no instantaneous switchover to backup power. Instead, the system took about 10 seconds to regain power. At which point, the camera system (the main component of the security system) would have had to reboot. When the power came back up, it lasted (as far as we know) for the remainder of the 12 hours, but it did not power the lights, there were no backup security lights, and all they had were a few feeble flashlights.
What were they missing? They clearly didn't have enough preventative or active security controls.
Bulletproof Windows: They just built a huge addition to their house, but they didn't bother to fortify the established areas of ingress. Once the windows are made difficult to penetrate with bars (on ALL the windows), you have to address the walls around them. Quarter inch thick steel embedded in the walls is a good start, but even that will not withstand a hit from a big vehicle (why don't more people Purge with bulldozers?)
Active Perimeter Controls: Electrified fences are a good start. Add in a few proximity mines (if they fall under the Class 4 weapon designation). From there, I'd suggest getting creative with pepper spray in the sprinkler system or even gasoline and an ignition system. The number one active control you can establish is motion tracking turrets. They aren't that expensive to build, and they can be one hell of a deterrent. Several enterprising youths have done proof of concept builds with paintball guns and airsoft guns, but the principle is the same.
Location: Once a year, for 12 hours, people want and can kill you without repercussion. Building a house in a crowded neighborhood is just not a great idea. You want your house to be in a place where people wouldn't take their children to trick-or-treat, as the ratio of distance to travel vs amount of candy is too great. A house in the middle of 20 mostly barren acres would be a good start.
Panic Room: Everyone I've talked to about this movie makes this observation. The nature of a layered defense approach is that you don't just have the barred windows to get past and then you're free to kill. Instead, you have to have multiple layers of defense. Here in the South, it's not uncommon to build a safe room in case of a tornado, and the principle is the same: a small, virtually impenetrable room where you can survive for short periods until help arrives.
In this particular scenario, Ethan should have built a steel-reinforced concrete bunker underneath the house with a vault-like door - just like you would if you were building a bomb shelter. This multi-layer approach would have significantly improved the chances of survival for the family. The suggestion that the system had been tested by security people and found to be 99% effective is laughable, but that might have been the boasting of the most successful regional salesman.
The movie itself was okay, but there are fundamental flaws with the world that they created.