Mobs track their movement based on what they perceive to be walkable terrain, not necessarily what is actually there.
Once you have your pistons, you can begin experimenting with different configurations and designs. Here are a few ideas to get you started: lovely craft piston secret mobs work
Create two sets of sticky pistons facing each other, separated by a gap of four blocks. Each set requires four pistons arranged in a 2x2 square. Mobs track their movement based on what they