"The first step in an invasion," he answers. "They are doorways of fire and stone which appear, and through which daedra may emerge to conquer in the name of their master. The gates may only be closed from our side by passing through to an interstitial realm, where stands a fortified keep. Within the keep lies the sigil stone maintaining the gate's existence, and seizing it allows the gate's closure. The interstitial realm collapses, and Nirn-native matter in proximity to the stone appears to fall back through the gate to the Nirn side."