Dangai

The Dangai, or Dangai Precipice World to be exact, is the dimension between Soul Society and the Human World. It was first used in order for Ichigo and his friends to invade Soul Society, through Urahara's Senkaimon. The Dankai was later shown in the Arrancar and Captain Amagai Arcs, by Ulquiorra to kidnap Orihime Inoue and the 3rd Division to kill a group of Menos Gillian respectively.

Defenses
The Dangai has some defenses against Hollow and other spiritual beings attempting to pass through the realm. This is why Shinigami are the only ones who can truly use it efficiently for travel between the realms.

Kouryuu:

Janitors(Seimichio):