If you fancy for Narnia, you probably already know the approach for Roadside, after that around 10min to Cedar Rouge and another 5min (150 meters) to Narnia.
The approach can be tricky, because the path is a bit weird, but let's try to explain:
Go out of Madiba roof to the direction of Spider on The Roof. You'll see a small slab (1.5m high maybe). Climb the slab, walk on the rock to the right along the rock. Then you'll see a passage on the left, take that and you'll open the door to Narnia.
Check the trail on the map for a visual clue.