I've always thought Tolkien usually used "leaguer" to mean not so much a siege as simply a containment or partition. So when they put a "leaguer" on Angband, they're not actively laying siege to the fortress, but they are preventing anyone from getting in or out. Likewise, the leaguer of Gondolin is a sort of self-imposed containment with the obvious purpose of keeping the city's location secret. No one can leave. I wouldn't characterize Gondolin as a military encampment.
Maybe Unabridged Oxford-boy (I saw that smug smile! [img]smilies/wink.gif[/img]) has more information -- my wimpy Webster's is no help.
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