The Diagonal route provides an attractive alternative to the more popular Kasteelsport route as a means to ascending the Camps Bay (west) side of Table Mountain. It starts from the Pipe Track and, as per its name, it rises diagonally across three buttresses: Porcupine, Jubilee and Barrier buttresses. The top of the route deposits you on the Back Table in the Valley of the Red Gods.
The Diagonal starts from the Pipe Track trail, which can be gained from either the parking lot at Kloof Nek or by driving down the M62 into Camps Bay, turning up Fiskaal Road and continuing up the hill as high as you can go. There you can park on the side of the road and head up a dirt trail
until you reach the Pipe Track.
The trailhead for the Diagonal is about half a mile away from the Kasteelsport trailhead, in the direction of Kloof Nek.
The trail heads up steeply through bushy terrain, eventually reaching some rock overhangs. Head right from here (left takes you to a some wet scrambling along a waterfall) and make your way along a trail that rounds Porcupine Buttress.
Once at the base of Porcupine Ravine, head up steeply on switchbacks until you spot a prominent cairn indicating that you're done climbing and should now traverse across Jubilee Buttress.
Before rounding Jubilee Buttress remember to look back for a fantastic view of the Front Table part of the mountain. Continue past Jubilee Buttress and across Barrier Buttress. Then the path heads up Barrier Ravine until you reach the top and the Valley of the Red Gods. From there it is a short walk to the Twelve Apostles Trail.
It's a relatively easy hike, so standard hiking fare should suit you well. Conditions can change quickly, and clouds can come in fast over the mountain, so rain gear to stay dry might be useful.