How do I manage to draw an ellipse over my existing art work?

The layers are visible. However, the layer subordinate is hidden. When you click the ">" on the left side of the layer, the subdocuments appear. Check the eye symbol at the beginning. Could it be hidden?

After you have drawn your eyes, nose and ears click on the empty space to the left of the layer that they are on. When you do a lock icon will appear which means that nothing on that layer can be moved or otherwise affected. You should also lock any other layers that are below the layer that you have locked. You should then create a new layer by clicking on the icon to the immediate left of the trash can icon on the bottom of the layers panel. Once you have done that click on the new layer and then whatever you draw will appear on that layer. You'll notice a colored square appears at the far right of the layer. That indicates that the selected art is on that layer. See screen shot.

By the way, I assume you the shape you want to draw is an "ellipse" which is an elongated circle. An "eclipse" is when the moon goes in front of the sun and you'll burn your eyes out if you look at it directly.