The Perfect Fortress
The Fortress was located in a key system for the defence of its sector and garrisoned by Emperor`s Children. It was beautifully crafted, from tank depots to Administratum buildings. However the whole population centre was part of the fortress. The civilians were meant to form milita should the fortress fall under attack. Even flower beds were mined and every balcony designed as a possible defence platform.
After the outbreak of the Horus Heresy the Imperial Army took heavy casualties in their assault on the fortress. However their assault was just a bait. The self-reliant Emperor`s Children left the fortress to wipe out the retreating attackers and were wiped out themselves by the surprise attack of the Raven Guard. The loyalist forces took possession of the fortress without civilian losses.
The location of the "Perfect Fortress", the planet Narsis, may be a reference to Fulgrim's notorious narcissism.