The Christians have been shelling the ancient and sunbleached ruins of Castle Beaufort, perched high on a hill south of town, and the soldiers will not let us proceed father. The crusader castle was built by that most Christian knight, Fulk of Anjou, 800 years ago to defend the road running west to Sidon and Tyre against his Moslem enemies.

Today it is the Palestinians of Fatah who hold the now-crumbling donjon and the Christian militiamen south of the Litani River lob a few shells into the castle from to time just to keep everyone on his toes.

South of here, in the narrow strip of territory between the Litani River and the Israeli border about nine miles away, the civil war that has ended in the rest of Lebanon continues.

The Christian militiamen, with the help of the Israelis, are trying to establish their control over the border villages. But with little more than 800 fighting men, they have not been able to decisively defeat the Palestinians and Moslem leftists in the region.

The hill country here, rolling away to the great, snow-capped shoulder of Mount Hermon in the east, was known as "Fatahland" before the Lebanese civil war because the Palestinians had unrestricted use of the are for raids against Israel.

When the Palestinians had to with draw most of their forces to the north to fight in Beirut, the Israelis swore that the Palestinians should never again enjoy such an advantage. That is why Israel is backing and supplying the Christian soldiers with arms to create a security belt along Isreal's northern frontier.

The Syrians, who have imposed a peace on the rest of Lebanon, have been warned that they must not move south of the Litani River or approach Israel's border without risking an Israeli invention. And since neither side south of the Litani seems to have the strength to roll up the other, the war goes on.

The Syrians, who recently moved a battalion of about 500 men and about 10 tanks into the area just north of the Litani River line, appear to be thin on the ground and under orders not to farther south.

One company is encamped here in tents on the outskirts of town. A Syrian officer tells us that his mission is to keep the peace between Moslem and Christian villagers north of the river.

He said his orders do not include disarming the Palestinians, who are here in strength, of their heavy weapons. But when and if such orders come, he said, he did not doubt the ability of his troops to carry them out.

The presence of the Syrians here is thought to have prevented the unrestricted use of the big, 155-mm howitzers, of which the Palestinians have about a dozen in this region. The guns of the Palestinians are heavier than anything on the Christian side and could be matched only be Israeli howitzers firing from within Israel. A heavy and unrestricted bombardment of Christian villages nestled close to the Israeli border could have provoked a stiff Israeli response.

Even so, the Israeli raised a diplomatic row recently over the fact that the Syrians had come even this far. The so-called "red line" that might trigger an Israeli response if crossed is defined by the Israelis not solely in terms of geography but as a combination of six factors. They are: the location of the force, the duration of its stay, and its size, national composition, weapons and intentions.

While towns and cities to the north are cleaning up the rubble of war and getting back to normal, life in the border villages still shows all the strains of continuing war. In Bint Jebel, one of the Palestinian-held towns south of the Litani, about three miles from the frontier, one can sense the anxiety as people hurry along the streets. There are shell holes in the streets and a few houses have gaping holes and shattered plaster. Villagers bring visitors bits of shrapnel with Hebrew lettering to prove the Isreali involvement in the border fighting.

Many shops are closed and shuttered. People have moved away, and the walls are covered with leftist slogans and posters. A Lebanese physician fighting with the Palestanians said that in the last three months there has been about 30 people killed in the area.

He said he had no hospital, no surgical facilities, no blood plasma, few drugs and no ambulance. He said, however, there were enough weapons and ammunition to successfully defend the town against a Christian attack.