@OP, the dual trap blocks are indeed unneeded. Make the tower more like this: Pretend that the wood door is a trap block and the dirt blocks are wood blocks. Alternate stone wood stone wood for the blocks under neath the spikes and put a backwall over each spike along with a trap block on top. Much more effective, less costly, and more deadly. Of course it's still a kinda bad trap anyway but I'm thinking more along the lines of "Your trap could just be a tad more effective."