Viewport - the main working space of the animator. You can use it to view scene objects and control them.
To control Viewport’s camera you need to hold Alt.
If you are on Linux and this combination is already used by your OS window manager, you will need to change them in the OS settings. For the time being these hotkeys are not customizable in Cascadeur.
You can see the Viewcube in the top right corner. It shows from which side you are viewing the scene. Different colors mark different axes.
Click on the cones around the Viewcube to view the scene from different sides.
You can see an object from different sides simultaneously by using several viewports. You can also quickly switch between two different viewports.