[Gwaihir the Windlord]

Three times Lorien had been assailed from Dol Guldur, but besides the valour of the elven people of that land, the power that dwelt there was too great for any to overcome, unless Sauron had come there himself.

This power, of course, was Galadriel's Nenya-produced 'time envelope' around the wood and all that went with it. It was indeed comparable to the Girdle of Melian -- although not as powerful, and over a much less distance. But is was the same sort of defensive thing.