You will get a door to a secret tower defense level after that mission. Push the panels in the following order: Bah, Ram, Ewe. When they are pushed, the corresponding sheep will say either 'Bah', 'Ram' or 'Ewe'. The room has three different colored sheep surrounded by energy. You need to bust through a door to get inside. Look closely around the middle of the map to find a room.
Around the horizontal center of the map, there is a winding corridor with several rooms you need to go into in order to get the power-ups and equipment.
In the third Human mission, where you need to escape a prison with the Elves, there is an entrance to a secret level.
