http://bit.ly/gq4YQu
These walls are built to withstand creepers! As the creeper explodes, the outer stone will be destroyed, then the inner sand will fall down, replacing the wall and not allowing anything else to come through.

Note that the top design is lighter, and anything within 3 blocks of the wall may be destroyed. The bottom one is thicker so it all but prevents that. The walls are built to be instantly refilled at the top as soon as the explosion is discovered, and the stone should be replaced at daytime.

I thought of this mainly for my server, which is centered around fighting monsters. Let me know if you're interested!

Happy holidays!