Location from Red Rising by Pierce Brown
Mercury's capital city and the primary theater of the devastating urban warfare that consumed Darrow's forces for years during the Republic era.
Heliopolis was built to survive Mercury. It was not built to survive the war that found it. The city's heavy construction — necessary to endure the planet's volcanic activity and solar exposure — made it a nightmare for attackers and a fortress for defenders. When Darrow's Republic forces came to break the last Society holdouts on Mercury, Heliopolis became the anvil they broke themselves against. Atlas au Raa turned every building into a fighting position, every tunnel into an ambush corridor, every block into a kill zone. The Ash Rain made air support unreliable. The city's underground networks let defenders appear and vanish at will. Republic legions bled for years taking ground that the enemy would simply reclaim through tunnel counterattacks the next week. Heliopolis taught Darrow that you can be the best general in the system and still lose a war of attrition when the enemy is willing to trade time for blood.
A fortified city built to withstand Mercury's extreme conditions — thick-walled buildings, underground transit networks, and environmental shielding against solar radiation and the constant Ash Rain. The streets are coated in grey-black volcanic particulate. Bombed-out blocks alternate with defended strongpoints. The skyline is a jagged silhouette of shattered towers.
Also known as: Heliopolis, Mercury capital, the Mercury front