The weather, incidentally, was fabulous. Sunlight streamed off the rocks, melting everything into a dazzling haze.

We continued our climb.

The second time we stopped, more desperately out of breath, I unhooked a water flask I was carrying on my belt. We sat side by side on a large rock, too hot to fight.

'What's the matter?' Something Helena had said earlier had struck a nerve. 'Finding out that I'm acting for the Chief Spy?'

'Anacrites!' she snorted with contempt.

'So? He's a slug, but no worse than the other slime lovers in Rome.'

'I thought at least you were working for Vespasian. You let me come all this way thinking that -'

'An oversight.' By this time I had convinced myself that was true, it just never came up in conversation. Anyway, what's the difference?'

'The difference is, Anacrites when he's acting independently is a threat to you. I don't trust the man.'

'Neither do I, so you can stop erupting.' Hauling her up here had been an inspired move; I could see she had lost her energy for bickering. I gave her more water. Then I kept her sitting on the rock. The soft sandstone made a tolerable backrest if your back was muscular; I leaned on the rock and made Helena lean against me. 'Look at the view and be friends with the man who loves you.'

'Oh him!' she scoffed.

There was one good thing about this argument: yesterday, when we left the outer caravanserai and entered Petra itself down the famous narrow gorge, we had been squabbling so bitterly none of the guards gave us a second glance. A man listening to his woman complaining about him can ride pretty well anywhere; armed retainers always treat him with sympathy. As they had waved us along the raised causeway and into the rocky cleft, then hurried us on under the monumental arch that marked the way, little did they know that at the same time as she harangued me Helena was reconnoitring their fortifications with eyes as sharp and a mind as acute as Caesar's.

We had already passed enough rock-hewn tombs, freestanding blocks with strange, stepped roofs, inscriptions and carved reliefs to strike a sense of awe. Then had come the forbidding gorge, along which I noticed sophisticated systems of water pipes.

'Pray it doesn't rain!' I muttered, as we lost sight of the entrance behind us. 'A torrent rushes down here, and people get swept away:'

Eventually the path had narrowed to a single gloomy track where the rocks seemed ready to meet above our heads; after that the gorge suddenly widened again and we glimpsed the sunlit facade of the Great Temple. Instead of exclaiming with delight Helena muttered, 'Our journey's superfluous. They could hold this entrance against an army, using just five men!'

Emerging through the crack in the rocks, we had drawn up abruptly in front of the temple, as we were intended to. Once I got my breath back from gasping with awe, I commented, 'I thought you were going to say, " Well, Marcus, you may never have shown me the Seven Wonders of the World, but at least you've brought me to the Eighth!" '

We stood in silence for a moment.

'I like the goddess in the round pavilion between the broken pediments,' said Helena.

'Those are what I call really smart entablatures,' answered, playing the architectural snob. 'What do you think is in the big orb on top of the goddess's pavilion?'

'Bath oils.'

'Of course!'

After a moment, Helena carried on where she had left off just before we reached this fabulous spectacle: 'So Petra lies in a mountain enclave. But there are other entrances? I had the impression this was the only one.' Dear gods, she was single-minded. Anacrites should be paying her instead of me.

Some Romans get away with treating their womenfolk like mindless ornaments, but I knew I stood no chance of that so I answered calmly, 'That's the impression the cautious Nabataeans 'ike to give. Now gape at the opulent rock carving, sweetheart, and try to look as if you just popped this side of the mountain to buy a pair of Indian earrings and a length of turquoise silk.'

'Don't mix me up with your previous trashy girlfriends!' she rounded on me crossly, as a Nabataean irregular who was obviously checking for suspicious faces wandered past. Helena took my point. 'I may buy a bale in its natural state, but I'll have it bleached a good plain white at home:'

We had passed muster. Easily fooled, these guards! Either that, or they were sentimentalists who could not bear to arrest a hen-pecked man.

There had not been, yesterday, much time for me to sort out what lay behind Helena's wrath. Nervous about how long we could keep up our act as innocent travellers, I had taken us very hastily into town along the dry dirt track that curved away past numerous cliffside tombs and temples. We noticed that although this was a desert, there were gardens everywhere. The Nabataeans possessed spring water, and made the most of conserving rainfall. For people still close to their nomadic roots, they were surprisingly fine engineers. All the same it mas a desert; when it did rain on our journey, a shower had covered our clothing with fine reddish dust, and when we combed our hair, black grit had worked in right to the scalp.

At the end of the track lay a settlement, with many fine houses and public buildings as well as a tightly packed lower-class habitation full of small square dwellings, each set behind its own walled courtyard. I had found us a room, at a price that showed the Petrans knew exactly what a room was worth in the middle of the desert. Then I spent the evening scouting the walls to the north and south of the city. They were nothing spectacular, for the Nabataeans had long preferred to make treaties rather than physically resist hostility – a trick made easier by their custom of offering to guide invading troops through the desert, then taking the longest, most difficult route so that the troops arrived at Petra too exhausted to start fighting. (Most armies lack Helena's stamina.)

She was looking at me now in a way that made her considerably more attractive than most armies. She was completely wrapped in stoles against the heat, so she looked cool, though I could feel her warmth as I held her against me. She smelled of sweet almond oil.

'This is a wonderful place,' she conceded. Her voice had dropped to a murmur. Those rich dark eyes of hers still flashed, but I had fallen in love with Helena when she was angry; she was well aware of the effect it still had on me. 'I certainly see the world with you.'

'That's generous.' I fought back, though with a familiar sense of imminent surrender. At even closer quarters our eyes met. Hers were not scathing at all when you knew her, but redolent of good humour and intelligence. 'Helena, are you following the local rule of suing for peace?'

'Better to safeguard what you have,' she agreed. 'It's a good Petran system.'

'Thanks.' I favour the laconic in negotiation. I hoped Helena had not heard of the Nabataeans' other political custom: sending away their won-over opponents with large quantities of treasure. The Falco purse, as usual, was not up to it.

'Yes, you can skip the exorbitant gifts,' she smiled, though I had said nothing.

Asserting my rights, I slid my other arm around her. It was accepted as a term in the treaty. I started to feel happy again.

The sun beat down on the glowing rocks, where huge clumps of dark tulips with dusty leaves clung tenaciously. The voices ahead of us had passed out of earshot. We were alone in the warm silence, in what seemed a not unfriendly place.

Helena and I had a history of friendly relations near the tops of famous mountains. Taking a girl to see a spectacular view has only one purpose, to my mind, and if a man can achieve the same purpose halfway up the hill, he saves some energy for better things. I gathered Helena closer and settled down to enjoy as much playful recreation as she was likely to allow us alongside a public footpath that might be frequented by stern-visaged priests.