Introduction: Simple Flush Piston Door
You can use this to hide your base or really anything you would like to use it for.
Step 1: What You Will Need
You will need: Unspecified amount of random blocks, 12 sticky pistons, 6 redstone dust, 2 redstone repeaters, and a source of redstone power (levers, buttons, redstone blocks, etc.).
Step 2: The Wall
Make or find a wall (It doesn't have to be iron) and break open a 2x2 square. This will be where the door is.
Step 3: Pistons
On the inside put four pistons on each side like this.
Add 2 pistons as shown, facing toward the door.
Step 5: The Building
Put two blocks in the space in between the pistons (First image) and then put seven blocks along the top (Second and third image).
Step 6: Redstone Dust
Add redstone along the top as shown.
Step 7: Redstone Repeaters
Add two redstone repeaters like this on two ticks and facing outwards.
Step 8: The Input
Now make an input connecting to one of the 2 pieces of redstone dust in the middle. Make sure it doesn't touch any of the pistons.
Step 9: Testing
Step 11: Troubleshooting
One lag I experienced was the delay for the redstone repeaters wasn't working so it pulled back the blocks but didn't pull them aside. How I solved it was I just move where the lever or button was. If it was on a block I would just remove the block. Hope this helped.