If you want your units to have access to the top of the cliff plateau, you will need to include a way for them to get there. The next steps will show you how. If you do not want to give units access, then just leave it the way it is. I sometimes do that and place an Oil Derrick or Tech Airport on top. Makes the game interesting.
Helicopters still have access.
And the Allies can use the Chronosphere to transport units to the top. In order for all units to have access to the cliff plateau, you'll need to place a ramp in your cliff wall. This is very easy to do, and the picture to your left shows you how. The pic shows you right where to find it. The ramp is made up of two pieces that should fit perfectly together.