Those spikes seem better equipped to take out tires than pursuers on foot...
Oh they work just fine against feet, as long as the spikes are long enough to get through the sole of your shoe. Even if you don't step directly on it but instead kind of knock it over with your toe, it'll just tip over and roll another spike right into the bottom of your foot.
And of course against bare feet, getting through the shoe isn't a problem.
They're also little terrors against horses.
A person who knows they're there can try to watch their step, but doing that slows you down. Win/win for someone trying to escape. Unless of course you just clear your path with magic. Because magic solves everything.
Of course, you also have to throw a *lot* of caltrops for them to be effective. Since you have to saturate a large enough area that your pursuer can't easily go around or jump over them. Though would *you* want to try jumping over a patch of caltrops and hope you don't land short?
Those little things are medieval landmines.
