If you line the pit shaft with cobblestone/concrete all the way down, and attach blade traps to the side of the shaft walls on all 4 sides , you create a giant meat grinder essentially. Grand Spartan has done this in a few of his videos. Whatever makes it past the grinder and lands in the...
You have to design the pit so it has a solid (ie. concrete) bottom and sides. By the time most zombies are done beating on the concrete they've already died to the spikes they're crawling around on. You do not want to leave your pit sides/bottom as stone. Design it right and they're not...