OK, let's go back to the original quotation:
Quote:
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... and when the Shadow passed, Celeborn came forth and led the host of Lórien over Anduin in many boats. They took Dol Guldur, and Galadriel threw down its walls and laid bare its pits, and the forest was cleansed.
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So Celeborn led the army of Lorien in an attack against the demoralised forces of Dol Guldur - demoralised because the Shadow had passed, ie Sauron had been defeated.
They took Dol Guldur, and
then Galadriel threw down its walls and laid bare its pits. So, the fortress was taken by the army of Lorien led by Celeborn before Galadriel did any throwing down or laying bare. Quite possibly she did use magic, or perhaps this was achieved physically by Lorein Elves under her direction. But either way, given that there was no longer anyone defending the fortress, it could be done at leisure.