You can create a new floating 3D-window, from the 'Layout menu', or the '3D windows' icons.

In the 3D window, you will be able to move around in the level, almost as if you were doing it using NOCLIP in the FPS-game.

To move around, you can use the mouse, or keys, when the mouse is inside the 3D-window so the window becomes active. The keys you can find and redefine in the '3D View' options.

Another feature of the 3D window is, that you can edit your map in it. You can click on a polyhedron, and it will appear selected with wireframes. You can move the selection around, just as you would do it on the map-views. Even align textures on faces can be done in the 3D window.

Once you have created a new 3D-window, a 'eye'-icon will appear in your map-views, showing where the camera are positioned in the level, and in what direction its pointing.

You can move this camera around, and adjust its direction-handle, just like any other entity. However the camera won't be a visible object in the 'Tree-view (hierarchy-view)', since it only exist when a 3D-window are open.

If you have more than one 3D-window opened, there will also be more 'eye'-icons in your map-views. One per 3D-window.