The Green Gate is the first of the Three Gates and is a curtain wall running across the Canyon the surface of which is covered in bronze spikes so that the whole mass looks like an immense hedge or a row of trees. At the lower level, gates act as ‘airlocks’ where goods from the outer world are transferred to the other side by Red Ichorians. Gates higher up on either side allow the master Leftway and the Lords’ Road to continue along the Canyon. The former allows messages to pass in and out of Osrakum. The latter is used by Masters.
The diagram also shows how the Cloaca passes under the Gate through grilles. A throng of people are shown for scale.
The second, plan diagram shows the double set of gates each consisting of three gates which seal off, between them, two ‘airlocks’. It is by this means that contamination from the outer world is kept out of Osrakum.