Located in the Tiamat star system in the Iserlohn Corridor, the one of two navigable star-routes between the Empire and the Free Planets Alliance (the other being the Phezzan Corridor), Iserlohn Fortress was a massive space-station encased in a sphere of liquid metal. Virtually impregnable, the Fortress kept the home territories of the Empire safe from FPA invasion.
Iserlohn Fortress had a diameter of 60 kilometers and a mass of nearly 60 trillion tons. It's primary weapon was an enormous x-ray cannon, named the Thor Hammer, which was capable of annihilating vast quantities of enemy ships in a single breath. Its docks could berth more than 16,000 ships. Its dedicated agriculture blocks could sustain both its garrison and a population of several million indefinitely. The fortress exterior is several layers of liquid metal, which absorbs solid projectiles and reflects directed energy. Hundreds of cannon turrets float in the liquid metal, and they rise to fire at approaching enemy.
When the Goldenbaum Empire discovered the existence of the Free Planets Alliance, they immediately decared the FPA to be a rebel government, and dispatched the Imperial Fleet to deal with the "rebels." At the time, there was only one known navigable route between the Galactic Empire and the Free Planets Alliance--through a narrow band of star systems that would later become known as the Iserlohn Corridor. Naturally seeing the signifcance of the choke point, the Empire constructed a massive space-fortress along the Corridor in Imperial Year 457, in orbit of the Tiamat Star. Based on the Geiersburg Fortress, they named it the Iserlohn Fortress, it would serve to both guard against the unlikely possibility of FPA warships entering Imperial territory, and serve as a base-of-operations for the Imperial Invasion through the corridor.
During the Goldenbaum era the Iserlohn Corridor was the only route connecting the FPA and the Empire, as Phezzan's autonomy was respected by both sides. As such it was the main obstacle to any FPA expedition against the Empire, and was put under siege many times, these sieges always ending with defeat and decimation of the Alliance fleet. Among the FPA Admirals that tried and failed against Iserlohn, Fleet Admiral Sitolet, who led the 5th Iserlohn Offensive, is still in active duty during the series. Sitolet showed ingenuity during his campaign against Iserlohn, determining the range of the Thor Hammer and then luring the Iserlohn Fleet outside its range. He then exercised a continuous pursuit, following the Iserlohn Fleet in firing range but always leaving it between his fleet and the Fortress, as to neutralize the Thor Hammer. His aim was to force the Imperial Fleet inside the Fortress, and then follow suit before anything could be fired at them. The plan eventually failed, because Sitolet did not have enough ships to sustain the pursuit long enough, but it was by far the most effective FPA campaign against Iserlohn.
Free Planets AllianceEdit
Yang Wen Li never considered Iserlohn Fortress the pinnacle of imperial power, but merely an over-reliance on technology. He understood the fortress's limitations from the 5th Iserlohn Offensive, led by Admiral Sitolet. Sitolet determined the precise ranges of the Thor Hammer, but his fleet was undermanned to carry out a decisive victory. Yang's tactic was the opposite, to infiltrate the fortress with the Rosen Ritter while the garrison fleet was chasing a decoy. The 7th Iserlohn Offensive, with merely 6000 ships, successfully won Iserlohn Fortress for the FPA. Yang hoped Iserlohn Fortress would be the FPA's bargaining posture in deciding a favorable treaty with the empire.
Yang Wen Li felt the fortress was disposable, since it was not at all a location of political or economical importance. When Goldenlohen Empire used the Phezzan Corridor to invade alliance territory, Iserlohn lost all strategic value. Yang deceptively "lost" the fortress to Admiral Reuenthal, but regained it from Admiral Lutz. Yang Wen Li is the only commander to truly conquer Iserlohn Fortress at all, as well as multiple times.