What's cheap and deadly?
Lots of things.
You can pound nails through a board and lay that on your outside windowsills if you don't want anyone parking their butt there. And you see this, even on windowsills five feet off the ground. Mostly it's more elegant — wrought iron spikes seem preferred in these parts as the proper reminder to keep moving.
And then there are other problems. Padlocks seal doors. Multi-fanged dogs discourage callers. Razor wire and high-voltage lines strung across wall tops attenuate the enthusiasm of most night-creepers.
If you have the money. Even dogs need to be fed.
Razor wire is a high-tech item here. Has to be crazy expensive. Electricity too, not considering the fuss of putting all those parts together after they've been bought. And remembering to turn the switch on when it needs to be on. Or was that the other way around? Let's see...oops — I guess I had it right the first time. Sorry, Alex. Alex?
To hit the high peak of recycling and the low valley of cheap, nothing can beat broken glass set in mortar on top of any wall whatsoever. I think. Probably so. Because the evidence is plain to see. It's used everywhere.
Have another beer or two and you've got your raw material. Put the bottles in a gunny sack, apply hammer, stuff your hands into work gloves, and pull the spears of death-dealing shattered glass out of the bag. Which cuts better than anything. Doesn't rust or corrode. Comes in colors. Is cheap.
Did I say cheap?
Can't repeat that too often. It's here and it's cheap. And the technology is well understood.
What could be better?