Select the 'Custom Campaign' option at the menu, then choose the 'Exodus Of The Horde'. 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.
