Eventually I rolled him over on his back and stood up again.

He was fortyish. Too fat and flabby. A wide, berry-brown face with a heavy chin and thuggish neck. The face looked mottled under its tan. Short arms; broad hands. He had not troubled himself with shaving today. Lank, rather long hair merged with coarse black eyebrows and dripped sluggishly on to the rock floor beneath him. He was dressed in a long, loose-weave brown tunic, with a more sun-bleached cloak tangled wetly around him. Shoes knotted on top of the foot, a toe-thong apiece. No weapon. Something bulky under his clothes at the waist, however – a writing tablet, not written on.

Helena held out something else she had found beside the cistern – a round-bottomed flask on a plaited leather cord. Its wicker casing, stained brown with wine, made me pull out the stopper: wine had been in it recently, though only a couple of drops shook out on to my palm. Maybe the goatskin had contained wine too. Being tipsy could explain how he came to be overpowered.

His attire was Eastern, protecting him from the burning heat. Those swathes of cloth would have impeded his movements if he had been struggling to escape an attacker. I had no doubt he had been attacked. His face was grazed and cut, probably where he had been pushed bodily over the edge of the water tank. Then someone must have jumped in alongside, probably not to hold his head under; marks on his neck looked more like strangulation to me. Helena showed me that in addition to the ground that had been soaked when I clambered out, beside the tank on the far side was a similar wet area where the killer must have emerged sopping wet, The sun had made his tracks faint already, but Helena had found them leading back towards the ceremonial platform.

We left the body and recrossed the summit in front of the altar. The trail petered out, already evaporated by sun and wind. To the north we found a moon god's shrine with two crescent-crowned pillars flanking a niche; beyond that lay a wide staircase leading downwards. But now we could hear voices approaching – a large number of people, intoning a low ceremonial chant. This was plainly a major ceremonial route to the High Place. I doubted whether the killer could have rushed down that way, or the procession now winding up the stairs would have been disturbed.

Helena and I turned away and climbed back by the same steps that had brought us up. We scrambled down as far as the priests' house or guardpost. We could have knocked and asked for help. Why take the easy way? Still loath to encounter anybody with a sharp implement who might view me as an easy catch for the altar, I convinced myself the murderer would have crept past anonymously too.

Now I noticed a second path. This must be the one he had taken; he had certainly not passed us while we were canoodling. Helena was a senator's daughter after all; she was supposed to know the meaning of modesty. We had been alert for voyeurs.

I never know when to leave well alone. 'Go down,' I commanded Helena. 'Either wait for me near the theatre, or I'll see you back at the lodging. Go down the same way we came up.'

She made no protest. The sight of the dead man's face must have stayed in her mind. Anyway, her attitude mirrored my own. I would have done this in Rome; being a visiting flea on the rump of civilisation changed nothing. Somebody had just killed this man, and I was going after whoever did it. Helena knew I had no choice. Helena would have come with me if she could cover ground as fast.

I touched her gently on the cheek and felt her fingers brush my wrist. Then without a second thought I started down the path.

This path was much less steep than the one we had come up by. It seemed to be heading into the city, a much longer way down. Sudden wicked turns forced me to watch my footing above astounding aerial vistas that would have made me quake if there had been time to look at them properly.

I was trying to be quiet as I hurried. Though I had no reason to think the fleeing man knew pursuit was hot on his tail, murderers rarely hang about studying the view.

I was passing through another valley gorge cut by watercourses, like the one that had brought Helena and me to the summit. Flights of steps, inscriptions in the cliff face, sharp corners and short stretches of narrow corridor led me downhill as far as a rock-carved lion. Five strides in length and pleasingly weathered, he served as a fountain; a straight channel brought fresh water down through a pipe and out of his mouth. Now I was certain the killer had come this way, for the sandstone ledge beneath the lion's head was damp, as if a man with wet clothing had sat there snatching a drink. I splashed water hastily over my own forehead, thanked the lion for his information, and rushed on again.

The water that had flowed through the lion now trickled downhill in a waist-high runnel cut into the cliff face, keeping me company. I stumbled down a steeply winding flight of steps then found myself in a secluded stretch of the wadi. Overhung by oleanders and tulips, its peaceful stillness nearly made me abandon my quest. But I hate murder. I strode on. The path came to a pleasant temple: two free-standing columns in a pilastered frame, with a shrine behind, darkly dug out of the mountain like a cavern. The portico was approached by wide steps, a parched garden in their base. There I saw an elderly Nabataean priest and a younger man, also a priest. I had the impression they had just come out from the temple sanctum. Both were gazing downhill.

My arrival made both of them gape at me instead. In Latin first, automatically, then in careful Greek I asked the elder man if anyone had just passed that way in a hurry. He merely stared at me. There was no way I could attempt the local Arabic tongue. Then the younger man suddenly spoke to him as if translating. I explained briskly that somebody had died at the High Place, apparently not by accident. This too was relayed, without much result. Impatient, I started walking on again. The elder priest spoke. The younger one came straight out from the garden, and loped downhill alongside me. He said nothing, but I accepted his company. Glancing back I saw that the other had turned to go to the place of sacrifice and investigate.

My new ally had a dark desert-dweller's skin and intense eyes. He was wearing a long white tunic that flapped around his ankles, but he managed to shift along pretty fast. Although he never spoke I felt we had shared motives. So, feeling slightly better than strangers, we hurried downhill together and eventually reached the city wall, far over in the western precincts, where the main habitation lay.

We had passed no one. Once we entered by the city gate there were people everywhere, and no way of discerning the man we sought. His clothes must be dry by now, as mine almost were. There seemed nothing else I could do. But the young man with me still strode ahead, so I found myself drawn along with him.

We had emerged close to the public monuments. Passing through an area of impressive homes built from well-dressed sandstone blocks, we reached the craftsmen's quarter on the main thoroughfare. The gravelled street cried out for decent paving and colonnades, but possessed its own exotic grandeur. Here, the great covered markets lay to our left, with an area of casual stalls and tethering posts between them. The main watercourse ran along-side this street, about ten feet below. Poky stairways ran down to that lower level, while handsome bridges spanned the gulley to reach important buildings on the far side – the royal palace, and one of the monumental temples that dominated this part of the city. These lay on wide terraces and were approached by spectacular flights of steps.