 In response to artillery's increased reach, military architects began in the 1850s to build detached polygonal forts farther away from cities to protect them from bombardment.To counter its new destructive power, architects began making those forts more durable.In 1887, French military engineer Henri-Louis-Philippe Mougin produced plans for a "fort of the future" ("Fort de l'Avenir"), calling for a mostly underground fortress built of concrete and armed with artillery in retractable steel gun turrets.