The Sand Trap
I have constructed a trap that uses sand, torches and nearly any other block. Simply build a staircase and build a wall around it. On the wall, at the height you want, place a torch, make it the same height on each stair, so the torches form a diagonal line. Then place sand blocks on the torches, sand blocks do not fall through torches if they are placed directly on them. Then place as many sand blocks as you want on the torches. To activate the trap, place a water block on the block next to the top torch, the water washes the torches away and the sand drops.
Place the water where the cobblestone on the wall is.
The sand trap shall appear in some of my constructions.
Torches and sand can make buildings as well.