She op'nd, but to shut Excel'd her power; the Gates wide op'n stood,
If the Gates of Hell are open, all the devils can follow Satan and get out. (I'm reminded of Pandora's box - when she opened the box she wasn't supposed to open, all the evils (except Hope) escaped and then represented afflictions cast upon humanity.
And thrice threefold the Gates; three folds were Brass, [ 645 ] Three Iron, three of Adamantine Rock, Impenetrable, impal'd with circling fire, Yet unconsum'd.
Milton is referring to the Nine Gates of Hell, which Dante presented in his Inferno (part of the Divine Comedy). See http://historylists.org/art/9-circles-of-hell-dantes-inferno.html