TWO hundred kilometres north-west of Kathmandu, 10 days walk from the nearest road-head and several centuries removed in time, the walled city of Lo Manthang has resisted the rude intrusion of outsiders since its battlements were first raised above the barren surrounding plain. Virtually on the border of Tibet, in country so wild and desolate the only foreigners to pass through the region were traders stepping through the high passes of the Himalaya, Lo Manthang and the surrounding area known as Mustang has long slumbered in isolation. A king saw to local rule, the villagers wrenched one crop a year from the earth, and lonely darkened monasteries resounded to the beat of drums and monks' chanting. And so it might have continued, but for a revolution in the Nepalese capital in 1990 which shook up the government, which in turn revised its trekking policies. Mustang, which had officially been off-limits and only ever penetrated by one or two Westerners, was to be opened up to a number of controlled trekking groups. The reason for this was two-fold. By limiting numbers, the ecological impact on a remote and undeveloped area could hopefully be contained, and by charging a US$500 fee for each trekker, funds could be raised for such practical aid projects as restoringdecaying monasteries and providing villages with drinking water. It was also decided to benefit from lessons learned on more traditional routes like Annapurna and Everest, which had been infested with trekkers ''living off the land'' with the consequent degenerate effects of deforestation and litter. Each group going to Mustang would have to attend a ''green'' briefing before setting off, would need to take all its own food and cooking fuel, would have to display its own rubbish in a bag at the end of the trek and be escorted by a police liaison officer throughout who would ensure no misbehaviour. Far from putting would-be trekkers off, the exclusive high price of the trek only served to attract those who felt that such an expensive item must be worth buying, while the ecological element caught the spirit of the decade and made visitors feel positively virtuous. The first trekking groups made their way north last spring, and by the end of the year the route had proved so popular the Ministry of Tourism has decided to double the quota for 1993 to 1,000. Time dictates that most people have to fly part of the way to Mustang, sliding beneath the towering peaks in a flimsy aircraft to Jomsom, 2,682 metres above sea level and a 10-day round trip away from Lo Manthang. The difference is noticeable immediately, for this is no longer the Nepal of lush rice paddies, quaint thatched villages and rhododendron forests, but of flat roofed houses and treeless landscapes characterised by sand and stone. As the trail bends northward along the Kali Gandaki river, the surrounding area gets even bleaker, with occasional houses and Buddhist shrines the only indication of any human influence. The Himalaya, which dominate every other trek in Nepal, fall behind as the way opens up towards Tibet, and the people on the roads are ruddy-cheeked with their hair in braids, and only speak Nepali, the national language, haltingly if at all. This almost lunar landscape gives no hint of the surprises it conceals. In the lower reaches of Mustang, around villages like Chele and Tangbe, sweet red apples grow in orchards which farmers must irrigate and nurture with constant care. The fruit is crisp and juicy and in complete contradiction to the dusty ground which does not seem capable of supporting anything. These and locally grown apricots are distilled to make a fine spirit - hopefully described as brandy by persuasive shopkeepers - smooth, mildly intoxicating and wonderfully relaxing. Unworldly though the Loba, as the people of Mustang call themselves, may seem, economics has pushed many beyond the frontiers of their homeland. With as little as one crop of wheat a year, near total deforestation and an increasing population, many Loba head south in the coldest months to trade in India. Nobody can say for sure when this migration started, but the hill people now make regular business trips to Ludhiana in the Punjab to buy and sell sweaters. They are regarded as astute businessmen and women and some have travelled as far afield as Bangkok and Hongkong when profit has been in the offing. With a large proportion of the men habitually away raising cash, another custom has been adopted for purely practical reasons. Life expectancy is short and infant mortality high, so the women frequently take a second or even a third husband to ensure continuation of the line, another workaday and necessary solution for clinging to an existence on the edge of the earth. After four days of ragged ups and downs through passes which can reach almost as high as 4,000 metres, Lo Manthang reveals itself, solid as a monolith and dominating the surrounding fields - a green sward in the summer but dry and inhospitable at all other times. While there are four monasteries in the L-shaped city, it is the four storeyed home of King Jigme Palbar Bista which is the paramount feature. Its sun-baked mud brick walls rear up immediately inside the main entrance to the city, and behind its solid doors lives the man whose word is law from here to four days' march away. No tin pot monarch, Rajah-Saheb threshes his own grain, mediates in local disputes and with the government in the capital, and will condescend to meet groups of foreign visitors when he is not otherwise engaged. Entrance to royal circles in Lo Manthang may be easier than elsewhere, but it is no less bound up with protocol. Slightly shy trekkers, wrapped up like Michelin Men, are meant to offer His Majesty a clean, new prayer scarf on hands held out with the palms uppermost and the thumbs tucked into the side. King Jigme takes this cloth and ties it gently round the donor's neck as a sign of welcome. A bowl of fruit and a bottle of medicinal rum should also be handed over, but these will not be returned. Conversation is limited, as the King does not speak English, but there is time to admire his collection of crockery, photographs of the famous and other impedimenta safely displayed in glass-fronted cabinets, while trying to avoid consuming tea seasonedwith salt and butter and ice cold pastries. Like British royalty, King Jigme is a fine judge of horse-flesh and fond of dogs, although the ululating Tibetan mastiffs he breeds on the first floor of his palace could probable eat a brace of Windsor corgis for breakfast. The King's subjects have grown used to the odd visitors to their city to the extent of now affording them only a curious glance while the children might well muster a timid ''hello''. Their homes, stables and cowsheds - so often intermixed - are criss-crossed by a maze of alleyways and passages, broken occasionally by a square with a tap-stand. The children attend a school just past the giant copper prayer wheels which stand at the main gate, although the young boys may be conscripted into the monastery to learn the prayers and chants of the Sakya sect, which predominates here. The order's distinctive grey, white and yellow stripes on the monasteries' red walls reflect the colours of the surrounding hills, blending religion into nature in this strange enigmatic land. It is tempting to draw an analogy between Mustang and Shangri La. So long hidden from Western eyes, concealed in the high Himalaya and imbued with mysterious rites in its holy places, Mustang might seem to qualify for Nirvana. But the stark truth is that living there entails a life of hardship and unremitting toil, from the King to the lowest tiller of the field. Trekkers - brought tea in bed by their Sherpas, protected by modern medicine and carrying little more than a camera - will not experience this. For them, Mustang remains a tableau vivant of what life must have been like in many parts of the world in anyagrarian society before industrialisation carried all before it. And if the trekking fees are properly used, the rules laid down to protect the environment strictly enforced, and the numbers of lens-faced visitors kept to sensible limits, there is no reason why Mustang cannot go on being as wild and beautiful as it has always been, with the added bonus of a better life for its inhabitants. Royal Nepal Airlines fly to Nepal three times weekly. Tiger Tops Mountain Travel International runs treks to Mustang in spring and autumn from US$2,660 including trek permit and return air fare to Kathmandu. Tel 997 1 415995, fax 977 1 414075.