The lack of good, large, clear and detailed pics of Orthanc on the internet is ridiculous for someone trying to make a good, detailed model of Orthanc. Fortunately, I didn't need as much detail for this small model, but it was still frustrating.
Here's a better view of the arms and platform. It took a while of racking my brain to find a simple, yet effective solution and here is the result: a 1x2 slope, on a 1x4 plate, on a 1x2 plate with on stud, stuck in a 1x2 technic brick. I'm quite happy with it.