The Armoured Giants
Ornithischians needed to defend themselves, and porcupine quills were not going to ever be enough when pitted against the speed, agility and powerful jaws of their Saurischian predators. Some of them like the Pachycephalosaurs evolved extraordinary spiked dome heads that acted as a battering ram against the belly of their rivals and predators.
Others became more and more armoured until they became slow moving fortresses that were practically impregnable.
But they were fortresses with a sting! Far from being merely passive, their tails show massive clubs that could act as leg-breaking weapons. The vertebrae of their tails were reinforced with rods that stiffened them and made them lethal weapons for any two legged theropod that tried their luck!