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How does one make an efficient kill room in Minecraft, that can survive creeper explosions?

So, I contained the spawner; I can probably augment it with some spawn rooms that will transport the mobs to the shafts dropping them the needed 22 blocks down to what is to be the room where I kill them. Then I'll have a creeper blow up and set all consecutive survivors free.

I know the walls must be built from obsidian, but how do I make the gap in it to swing my sword through? The text tutorials are rather sketchy, and the video tutorials use either fenceposts (which are definitely not blast proof) or some kind of transparent grid which I failed to identify (a different tileset, maybe?).

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3 Answers 3

up vote 6 down vote accepted

If a creeper is standing in water, then the blast damage is restricted to the water. So you can have water for the mobs to stand in, and if one accidentally blows up then you're not totally screwed with a broken trap, but it will kill the other mobs in the trap.

This is mine: enter image description here

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How does this work with a 'drop' type trap? Wouldn't the water break the mob's fall? –  fredley Mar 6 '13 at 12:05
...and that thing above - is that metal bars or what? (I don't know your tileset). –  SF. Mar 6 '13 at 12:13
Pretty....what texture mod are you using? Me likey. –  mikeTheLiar Mar 6 '13 at 16:17
@fredley it does. You can use two drops. –  Kenneth Posey Mar 7 '13 at 6:56
@SF. Yes, it's iron bars. –  Kenneth Posey Mar 7 '13 at 6:58

A properly constructed XP grinder doesn't need to be made of obsidian, because the mobs will never be able to 'see' you for the purposes of attacking/blowing up.

The trick to constructing such a grinder is to have the mobs' feet on the same level as your head, with this level having the only opening and also having one block of horizontal space between you and the mobs.

Here's a side-on view of such an XP grinder

Mobs drop in here

| |
| |
| |
|||_ <- You stand here

For a video tutorial, check out this one by Monkeyfarm.

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Are the 14 repeaters and 8 pistons necessary? –  SF. Mar 6 '13 at 10:55
only if you want all the functionality, if you don't care about the crusher or on/off functionality then you can get away with only using 2 pistons for the loot pusher –  Steven Cunningham Mar 6 '13 at 11:09
This works well, but I've still had creepers blast on me. Seems to happen when I have a really full room (2x2) and they are glitching out. –  Hawkeye Parker Jul 25 at 8:22

Creeper sight through glass will not trigger an explosion when it "sees" the player. Often above the 1x1 or 1x2 horizontal opening used in mob spawners is a row of glass. This is not necessary for functionality but may better suit player aesthetics.

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