Encouraged by Roberts, Bonington conceived of the idea of climbing Annapurna by its South Face which he realised was going to require not just rock climbing but also siege climbing tactics because the ascent would take many weeks. No one had tried rock climbing at high altitude before. The face itself was almost twice as high as the Eigerwand and even above the rock band there was another to the summit. The topography near the summit could be assessed from photographs Bonington had taken on the Annapurna II expedition of ten years earlier.