From what I've read impaling was a common practice that Vlad ...um..."improved". Apparently the original practice involved running a sharpened stake through the abdomen front to back. Vlad didn't bother with sharpening the stake and he used an existing opening, planted the stake in the ground and allowed the weight of the body to do the rest....over time. No wonder the muslims were terrified of him.
In the movie, which seemed historically accurate, the procedure involved placing a sharpened, hollow spearhead on a stout (fencepost-sized) stake, then lying the stake across rollers and placing the victim at the point of the spearhead. A large man with a sledgehammer would then drive the stake between the victim's spread legs, entering roughly around the perineal area. The goal was to keep the victim alive until the stake pierced the other end of the body, preferably through the skull.
Nice, eh?