Who did it? Bucky, Crowley, Xayah, probably also various other people at the Invincible
What'd they do? Boarding up windows at the Invincible, blocking upstairs windows with furniture and more boards, creating barricades in the stairwells and halls of the upstairs floors, and on internal doorways on the main floor, to create prepared areas to fall back to.
Where'd it happen? The Invincible, within the first couple of days of the event.