Arabia's secret. In the course of one generation, ancient North Yemen has time-traveled into the 20th century; yet to much of the world it remains

North Yemen is a country that has kept to itself like nobody's business. Until recent times, this small, ancient nation in the southern corner of the Arabian Peninsula was a country of walled cities, as closed to tourism as Tibet once was. For centuries, the country's somewhat fanatical religious rulers, the imams, thwarted any incursions from would-be conquerors. After a protracted civil war in the 1960s and '70s, however, North Yemen (Yemen Arab Republic) began a remarkable transformation: In the course of one generation, a civilization comparable to that of Europe in the Middle Ages has become an almost modern country, one whose oil wealth and resources have boosted its strategic profile in the region.

Today, starkly beautiful North Yemen has rolled out the welcome mat to the outside world.

And despite the vast changes that have swept through Yemeni society - radios, televisions, and automobiles are in use in even the most remote villages, and some Yemenis have generators for water pumps as well - to the Western visitor it is a relatively pristine culture in which fascinating ancient traditions still hold sway.

Yemen is, after all, an ancient country. Archaeological exploration of its high mountain terrain has barely begun, but it is known that the region was on the edge of the southern and western desert caravan route extending from Oman to Syria.

Biblical connections are a part of the country's lore: The spices brought to the infant Jesus, frankincense and myrrh, grow only in the Hadramahut region of Yemen - so some biblical scholars speculate that the wise men may have started their journey to Bethlehem from this area (though, in fact, they could have merely traveled through here from somewhere else). The famous Queen of Sheba, who visited King Solomon, supposedly came from Marib, a ruined city 100 miles east of Sanaa, which has been undergoing archaeological excavation since 1951.

Sanaa, the capital of North Yemen, was once enclosed by walls with eight gates, and its old section is one of the few remaining places where a traveler can see how an Arab city looked before it was overtaken by modern times. Narrow streets are lined with centuries-old, multistoried stone houses. Muslims answering the daily calls to prayer worship in old mosques, some of which were built during the lifetime of the prophet Mohammad. Islam is believed to have been brought here by the prophet's contemporaries in the 6th century AD.

Because of North Yemen's fierce resistance to foreign dominance (even the Turks, no slouches in the business of conquering, ruled for only short periods in the 16th and 19th centuries), the country missed out on some of advantages enjoyed by South Yemen, its Marxist neighbor. Now allied to the Soviet Union, South Yemen (People's Democratic Republic of Yemen) was a British colony, called Aden, from 1839 to 1967.

Because of the British educational system, exposure to world trade, and more modern business methods, the country produced highly trained people during the period of British rule. When the communists took power in Aden, however, some of these trained people fled to North Yemen and today are high-ranking government officials and wealthy businessmen in Sanaa.

This accounts in part for the surprisingly widespread acquaintance with the English language in North Yemen. ``Welcome'' is one of the first words of English that a visitor hears, and it is sincerely meant and usually accompanied by a smile.