Suggestion: Vertical Field Array

Nice, localized force fields rather a bubble sound cool.

I like this too -- I had a similar idea:

Basically, a combination of the field generator and the magnetizer. You have a trench of buried pieces like the fence (but horizontal); these pieces, when EU-powered, would generate a force-field up to 10-20 blocks high. Another block like the magnetizer would provide power to the fence-pieces.

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In either design, the same general idea: A force-field without a ceiling that one needs to set out in advance instead of the bubble. Granted, one would need to encase these blocks in either obsidian or reinforced stone in order to protect them from attacks -- something the field generator doesn't have to worry about -- and have more material investment to lay out the field and power it, but you get the benefit of planning out your stronghold with the field in mind. Hopefully the power requirements would be slightly less to account for this.

Also, in either design you have the fact that you can tower yourself over the fence -- or use a jet-pack. A benefit in SSP, but a possible liability in SMP.