Mapping:Adding a camera to your map
A portal consists of 3 to 4 objects.
First you have an object is a brush you want to be the "viewer".
Then you have 2 entities, the misc_portal_surface and the misc_portal_camera.
This tutorial also uses an entity of target_position which is used to aim your camera, although you can give your misc_portal_camera an angle to aim instead.
Images
Download the following images and store these in your images folder.