This trap is super easy to make and is inescapable on servers with no /home, /spawn, /tpa or any command of the sort, or servers that do not allow ender-pearls. This trap is super easy to make and requires no knowledge of redstone.
Step 1: Prepare the Land
Dig a 4x4 hole that is 2 deep. Now get enough sticky-pistons for the perimeter and redstone to fill in the remainder
Step 2: Basic Redstone
Now place sticky-pistons along the perimeter, and fill in the remainder of the hole with redstone.
Step 3: Concealment, Holds, and Triggers
Now fill in the hole with any block, preferably stone or oak planks.
Do you still have some of that material you used for the floors? Make some pressure-plates to fill in one block from the outside.
Finally, go to each of the corners, and go one block diagonally. Place a 3 high pillar for each of the corners and add a small stop to prevent people from jumping over.
If you want to, you could add a ceiling to make it even more inescapable...
You are now Finished.