Have you ever made a fort on a server in minecraft only to for it to be destroyed in a matter of minutes? Well this wall could be the answer - it can be destroyed but then it rebuilds itself!
So to make your near invincible wall follow these instructions! First make a row of pistons facing upwards and connect them to a timer, then surround them with non-flammable blocks.
Put water on one side and lava on the other (the water has a one block "slope") to make a row of cobble-generators.
Step 3: Done!
Then activate the timer by switching the lever quickly on then off and it will start!
Because the cobblestone generates randomly you may want to make the timer longer. just add more repeaters!
If you want to make it so you can switch it on and off or want to reduce lag (or both) then you can add a sticky piston which pushes a block up and pulls it down with another lever. Ta Da!