A Window is a type of magical doorway that can lead to another world, or even another universe. They are constructed by mages using complex spells and can take a few hours to make. They are created by pure magical energy that are actually parts of the Old Ones and when the spell is in progress, these lights will come together slowly to form a window. Only one known person has been known to construct a window by moving these lights into position, Kernel Fleck.
When a Window opens, it manifests as a panel of light which varies in colour and was described twice in Demon Thief as having jagged edges.
Windows are the only way of moving from world to world in the Demonata universe. They can be constructed anywhere there and even in the Human universe. Anyone can step through them, human or Demonata, and if a Window opens between the Demonata universe and the Human universe, magical energy will seep through into the Human universe.
A Tunnel is a type of Window. It is only constructed with the use of a lodestone and a sacrifice. This sacrifice will create a bond between the person who killed the sacrifice and the tunnel itself. Once the killer has been killed, the tunnel will collapse in on itself leaving the Demonata vulnerable and weak.
A Tunnel can be used to allow passageway for the more powerful Demon Masters to come into the universe, as they cannot travel through the standard Window due to immense size and/or power.