Hellen replied to my query about the "door" in City at the Edge of the World:
He eliminated it by tuning his opening devise to field's frequency, which is exactly the way to induce resonance waves, which can destroy things. Well, at lest *sounds* reasonable idea...
I was sort of wondering whether one could do anything similar to the Liberator's shields? cf Iain's remark:
It is possible, that the shields work the same way - they allow the ship's lasers (with a particular frequency) to fire *out*, but stop everything else (with any other frequency) from penetrating *in*.