The name "Eleghost" refers to both the realm itself, and the single fortress-city Oberon built around the portal to the Nexus.
If you happen to be living in Eleghost, the single most important thing to know about is the threat index - a measure of how dangerous the local environment is right now. Indicator lights are everywhere, and broken indicators are a priority for repairs. An increase in threat index takes no more than thirty seconds, and is accompanied by an array of alarms.
Green: The rarest (normal) threat level. Green means that nothing particularly dangerous is within sight of the city. All gates are open; internal defense systems that could harm citizens are inactive.
Yellow: Normal. Secondary gates sealed; all inter-section traffic must proceed through one of the well-guarded main gates. Lightning moats and other dangerous but largely passive defenses are online.
Orange: Also fairly normal; orange means that the outer wall has been breached, or is in danger of being breached. This threat level is always active if flying monsters are in sight. Main gates are closed; all entrances to the portal room are sealed. All non-destructive defenses are online, including automated kill clanks at every gate.
Red: Inner wall has been breached, or is in danger of being breached. All defenses are online, including self-destruct systems and the three boom guns mounted around the central tower.
Black: Central tower has been breached, or is in danger of being breached. Nexus cannon is online.
The nexus cannon is an enormous energy weapon, built in the Nexus. It points directly at the portal to Eleghost, and can only be aimed by adjusting the portal. Firing it does, of course, wreck large portions of the city - but, at threat level black, it can hardly get much worse. The cannon has been used only three times: Once, to clear the area around the portal so the city of Eleghost could be built in the first place (five shots). A second time, when it was used to destroy a forty foot tall golden-age war clank (two shots). And a third time, when a shadow beast managed to sneak through most of the city and was about to enter the Nexus (one shot).
No building in Eleghost has a basement, and few provisions have been made for burrowing attackers; the entire city is constructed upon a massive and apparently indestructible gear - generally believed to have been imported from Tulthin. Even the nexus cannon is unable to damage this relic.