Well, that proposal is for those players with a lot of obsidian. The obsidian in that 'recipie' could be replaced with cobblestone - I chose obsidian as it is blastproof and takes a while to dig through, so if a player tried to dig their way out whilst in the lava layer, it would take a while.
Here's another plan - make an entrance hall out of chests. Dig down, make an underground house, and make stairs up to the entrance. It's hard to picture, but like this: (the gold shows where the stairs should be, and the wooden planks = chests).
Remember to make a door too. Lock all the chests and doors. This is important. The entrance is now impenetrable. Now, as you go down the stairs (NOTE: these mock-ups were quickly made in Classic, so I can't picture the stairs. But they are supposed to be the stairs that are like
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: blocks, you know).
Dig down a bit and place your stairs. Make a room down below. It doesn't matter how big, but preferably small. Then, at the end of the room, make another room. Make it bigger, and sealed off by a locked door. This is your actual house. Feel free to make a 'skylight' in that room, like I discussed earlier.
Now, this is the important part. Make the whole floor of the underground entrance hall stone pressure plates. It is important that they are stone. Wire a select few up to TNT. Make it so that only you know which plates are safe and which ones are wired.
Now, if a trespasser manages to get down there, they will step on the plates and cause an explosion. Make sure the explosion will not destroy the other room - the important one. It is absolutely foolproof. This means that by some sheer chance a griefer gets down there, they will be destroyed. By combining all of my ideas, you can make an impenetrable fortress.
The stairs are integral... Because the stairs are a different height to normal blocks, and if the whole floor is covered with pressure plates, it will look like the room has just a normal stone floor. Having the stairs means that a player cannot judge if the floor is at a different height, as pressure plates make the floor slightly higher.
Thanks.