I reset my SMP server a few days ago and now I'm working on rebuilding my castle entirely out of obsidian.
The good news is, I've thought up a new trap. It doesn't look as suspicious as my previous design (An upside-down torch-gravel mechanism powered pyramid balanced on a piece of iron ore) but I haven't tested it yet (although I have tested its invidual mechanisms)
Here's the plan on how to build it:
1. Dig four blocks straight down.
B = Any block, but preferably something with can't withstand explosions at all. Dirt, sand, netherrack, cake...
. = Empty space
. . . . .
. . . . .
B B . B B
B B . B B
B B . B B
B B . B B
B B B B B
2. fill up the hole like so, except for the top, which is left empty.
_ = Wooden pressure plate. Don't know if stone ones work.
! = TNT.
. . . . .
. . . . .
B B . B B
B B _ B B
B B B B B
B B ! B B
B B B B B
3. Sit your bait on top of the hole -- This can be any desirable block but preferably something which doesn't look artificial. For example, pumpkins in dirt areas, clay in sand areas, iron ore if you can't find either of the above, and gold ore if the trap's deep underground.
% = Bait.
. . . . .
. . % . .
B B . B B
B B _ B B
B B B B B
B B ! B B
B B B B B
4. Now put either sand or gravel on top of THAT.
G = Gravel/sand.
. . G . .
. . % . .
B B . B B
B B _ B B
B B B B B
B B ! B B
B B B B B
5. Build in the terrain on one side however you like to make it look natural.
. . G B B
. . % B B
B B . B B
B B _ B B
B B B B B
B B ! B B
B B B B B
VARIATIONS:
-- If you're too lazy to go to all the effort of collecting TNT, do something like this instead:
L = Lava.
. . . B B
. . G L B
. . G L B
. . % B B
B B . B B
B B . B B
B B B B B
-- Digging a deep pit of doom (with care taken to leave a layer of blocks between it and the TNT) underneath the mechanism will make it more lethal (plus you get to take the victim's stuff!).
. . G B B
. . % B B
B B . B B
B B _ B B
B B B B B
B B ! B B
B B B B B
B . . . B
B . . . B
B . . . B