The rope being tied around my waist by a monk comprised lengths of scraggly old leather attached to each other by granny knots. I have a fear of heights, and as I looked up, the cliff seemed to topple towards me by degrees. But this was the only way to visit the 6th century monastery of Debre Damo, which perches on a plateau in northern Ethiopia, a true wilderness. The monk whistled and I was plucked off terra firma like a sprat on a hook, yanked 50ft feet up the rock face by muscle power alone. Blessedly it was over within a minute, my terror to be replaced instantly with tranquility. I could see for 20 miles across a tawny landscape of desert and mountain. A bell rang faintly, and the vivid purple blossom on the trees had a pleasant fragrance. The church itself was an antediluvian hunch of masonry and desiccated beams protruding from the walls. Its interior, musty and redolent with age, was lined with hand-carved panels and millennia-old murals. The monks inside were almost as ancient, gnarled holy men clad in fez and robes, and whose smiles revealed toothless mouths. They read from handwritten codexes, the names of saints picked out in scarlet. Even getting to this place had been unnerving. The Foreign Office recommends against travel to within six miles of the Eritrean border due to banditry – but that is where you will find Debre Damo, one of the oldest churches on Earth. And the monastery is a crucial location in my historical thriller, Foretold By Thunder. I had to see it.