To quote from the Bill King short story, the part of it that could pertain to "Saturnine".
Reinforced by this fresh wave of troops, the hordes swept on, driving through the exhausted and demoralised Imperial troops to the very walls of the Emperor's palace. Khornate warriors howling their bestial war cries raced towards the marble and steel outer ring. Hordes of unstoppable Thousand Sons marched relentlessly forward, bolter fire raking the defenders. Slaaneshi Noise Marines swept aside the Imperial Guard infantry and reached the Saturnine Gate. Round the walls bitter fighting ensued as the Imperial soldiers sallied forth, trying to drive the attackers back before the main body of the assaulting troops arrived. Men died in their thousands. From pillbox emplacements in the palace walls Imperial gun crews rained death down on the relentless attackers. Again and again the streets outside the palace were swept clear of heretics. Again and again new foes stepped forward to take their place.Now indeed it seemed that the tide of battle had turned against the Emperor. The spaceports were firmly in the grasp of the minions of the Warmaster. Hundreds of thousands of troops poured down from orbit. Gibbering mutants and hideous amorphous Chaos Spawn surged out of the dread ships. Under the banner of the great eye, the sign of Horus, the lackeys of the four great powers of Chaos marched united. Mounted in Rhinos, lurking within mighty behemoths and clinging to the sides of gigantic war-engines, they made their way en masse to the Emperor's palace.Looking down on the seething sea of foulness, the defenders' hearts went cold. Mingling with the daemons and the mad-eyed cultists and the mutants, they could see heretical Space Marines and traitor Guardsmen. These were people they might have once fought alongside, who had once been as loyal to the Emperor as themselves. They looked upon a dark mirror of their souls. Down there they could see martial honour become berserk madness, human cleverness become sly treachery, hope become foulness and love become abominable lust. The brave men on the walls knew that there was no way out. Here they must stand and fight and die. There would be no mercy from those below.This was a war where there could be no honourable peace. It was destroy or be destroyed. For a moment all was silence, then Angron strode forth. In his brazen voice he demanded that the loyalists surrender. He told them that their cause was hopeless, as they faced a foe which could not be defeated. They were cut off, outnumbered, and defending a ruler too weak to be worthy of their loyalty. In that moment the men on the walls felt their resolve weaken. Looking at the transformed face of the Primarch who had once been one of the Emperor's finest warriors, they saw an invincible, relentless foe backed by a numberless horde and all the daemonic might of Chaos.There was a clamour on the walls as Sanguinius and the Blood Angels arrived. Standing on the wall, the angel-winged Primarch glared on Angron with angry contempt. For long moments their gazes locked, each Primarch seemed to be measuring the other, searching for chinks in the armour, for any sign of weakness and lack of resolve. Who knows what they saw there? Perhaps they communicated telepathically, brother Primarch to brother Primarch. The truth will never be known. Eventually Angron turned and walked back to his lines. He told his troops that there would be no surrender; they should kill everyone they found within the palace. No stone should be left upon stone.With a roar the horde advanced towards the walls. Great Lords of Battle lurched forward on iron wheels, crushing anything in their way, unloading racks of missiles and turning the area on the top of the walls into blazing storms of death. Doom Burners sent tongues of superheated metal licking out at the emplacements. Molten brass filtered through the windows and scalded those inside. Multi-tracked Cauldrons of Blood squirted jets of obscene daemonic ichor onto the defenders. Enormous Flesh Hounds of Khorne loped forward in their wake. Titans armed with specially constructed siege weapons lumbered into position. Battle cruisers dropped megatons of explosive death onto the defenders.Every loyal warrior knew that he was already dead, that there was no way he could survive the coming of the daemonic army. The soldiers fought with the desperate ferocity of hopeless men, firing until their weapons were empty, snatching up the bolters of the fallen, and facing monsters with the butts of their guns when all ammunition was exhausted. Three times the horde managed to scale the walls, and three times it was driven off by the valiant efforts of Sanguinius and the Blood Angels. Wearily the Primarch marshalled the defenders, rallying the broken, speaking words of comfort to the mortally wounded, fighting with cold, implacable fury when he was called upon to do so. Slowly though, despite his efforts, the Chaos forces managed to erode the defence. They seemed numberless as the grains of sand on a sea shore and Horus spent their lives carelessly.Outside the walls, Imperial forces frantically raced from their bastions to try to relieve the palace. Titan legions boldly cut their way towards the centre of the rebel army. The White Scars Space Marines harried its flanks. No attempt to break the rebel line succeeded. Breaking through that blood-mad horde was a near impossible task. All four of the daemonic Primarchs inspired their followers to feats of fiendish bravery. For every Chaos warrior who died it seemed that two more stood ready to take his place.In orbit, the Warmaster watched approvingly. If the palace fell and the Emperor died, loyalist legions across the galaxy would lose heart and the war would be over. Without the psychic shield of the Emperor's power, Humanity would swiftly fall prey to Chaos. Horus would stand triumphant amid the rubble of Humanity's greatest empire. He would become a new and angry god. If he did not win soon, reinforcements would filter in from the corners of the Imperium, and his attack would falter. For the Warmaster this was the desperate, ultimate gamble. Everything was staked on this attack. It had to succeed, and at that moment it looked as if it might.Day by day the siege wore on, casualties rose from the thousands to tens of thousands to hundreds of thousands. Bodies had to be bulldozed from the access ways to the Saturnine Gate by war machines. Chaos Titans blazed at the walls, specially constructed missiles ripping great chunks from the masonry. The Titans of the Fire Wasps answered their fire with volcano cannons. The smell of burning flesh filled the air as the corpses of the dead were incinerated in funeral pyres a hundred foot high. Obscene ash parched the throats of the defenders. The World Eaters built a pyramid of scorched skulls sixty foot high in Temple Square. By night the chants of degenerate cultists echoed through the streets and daemons flitted among the ruins of Earth.Slowly, foot by torturous foot, the defenders were forced back. The great walls of the palace were riddled with hundreds of kilometres of bulkheads and corridor. Within this maze, bitter hand-to-hand fighting ensued until entire sections of passage were filled with bloated corpses. Feeling that progress was too slow, Horus ordered the Titans of the Death's Head Legion to demolish entire sections of the wall. Despite taking tremendous casualties, the great Warlord Titans broke through, and the forces of the Warmaster flooded into the palace grounds.