Requiem was the very first Shield World, created even before the Human-Forerunner war. Shield-0459, the one from Halo Wars, seems to have been the more common type of Shield World that went into mass-production, and Trevelyan was special because it was intended to be the new Forerunner Capital (additionally, it was the sixth Shield World, so it would have been constructed during the political conflict between the Didact and Faber, so the Didact went all-out on designing this one to try and sway the Ecumene Council). The Flood likely invaded the Shield-0459's interior to take refuge from the firing of the Halos, and then recolonised as much of the exterior as they could.