If you arrive in Canterbury by train, you are almost immediately aware of the cathedral, there beyond the rooftops. To say it dominates the city is an understatement. From this distance, the pinnacled central tower is undoubtedly the vertical climax of the building - and the city around it.
Known as the ''Bell Harry,'' this tower, built in 1500, was the final crowning glory of an architectural development that began after a fire in 1067, the year after William the Conqueror's landing. Up to 1067 it had been a Saxon cathedral built on a Roman site. But then this old building had to be completely replaced.
From the station, you thread the intervening streets and pass through a Tudor gateway (as if entering a college at Oxford or Cambridge). Then you come suddenly on a church whose overwhelming effect is not, after all, in its height - but in its length - for which we can thank the 12th-century masons, William of Sens, and his successor, William the Englishman.
If fantasy can cope with the notion, the cathedral's amazing extension from west to east gives it the air of an ocean liner in dry dock - but a great vessel of Caen stone constructed of buttresses and windows and roofs, of multiple side chapels and transepts, of arcading, towers, and porches. What you see is an elongated mixture of church architecture of contrasting periods and styles which have somehow achieved a unified character in spite of their differences - by a mysterious combination, perhaps, of compromise, familiarity, and the mellowing effect of standing for centuries in the same Kentish air, on the same green site.
The only features of the main fabric which refuse to merge are the clean instances of recent work - for fresh conservation and repair are continually under way. Time will presumably soften it, too.
It does take time for the visitor to grasp the long complexity of Canterbury. Inside, it is not much easier become oriented.
This makes the excellent plan sold in the cathedral bookshop a must. Or, even better, the services of one of the cathedral's devoted guides.
You couldn't possibly have a more helpful one than Evelyn Ellender. This quickly informative and cheerful woman, crisp-spoken, with a catchy laugh, sat us down in the presbytery and treated us to an ''encapsulated'' history, skipping through the centuries with scarcely a breath.
It wasn't long before she arrived at Thomas a Becket, of course - the ''holy blissful martyr,'' as Chaucer put it in ''Canterbury Tales.'' Becket's gruesome end in 1170 and his subsequent and almost immediate veneration as popular focus for pilgrimage are indelibly interwoven with the growth and character of the cathedral.
''It's hard to convey to people,'' said Mrs. Ellender, ''the attitude of the Middle Ages. Those were the days when they loved relics. Every altar had to have its relics! You have to understand that, to understand the church and how it was built.''
It was his shrine that became the focal point of the new Trinity Chapel. His remains were moved there, or ''translated,'' in 1220, with spectacular ceremony and pomp, from their previous place in the Norman crypt below. It was eventually destroyed in 1538 (Thomas a Becket being first tried posthumously as a traitor), not long after Henry VIII had repudiated papal jurisdiction in England.
But for 31/2 centuries, until the Reformation, pilgrims had flocked in the thousands to Canterbury and the martyr's shrine. Becket's shrine was the main purpose in their coming - though there were other shrines also favored, like those of St. Dunstan, one of the early Saxon archbishops, and St. Alphege.
In certain respects, spring is Canterbury's time of year. It was spring in AD 597, for instance, when Augustine reintroduced Christianity to Britain, starting worship in Canterbury. And April, according to Chaucer, at the end of the 14th century, was the month for pilgrimages. Were they simply out for a good time in pleasant company - a walking holiday? Was ''Caunterbury'' (as Chaucer spelled it) just a stopping place for tourists? Certainly the cathedral, with its splendid glass windows (some call them the finest late 12th- and early 13 th-century glass in England), is brilliant on a sunny spring day. Tourists of the 1980s, no less than those of the 1380s, find sun pouring into the cathedral irresistibly beautiful.
But at least some of the medieval pilgrims, according to Chaucer, were devout in purpose. Becket was not, in fact, specially noted in his lifetime for holiness, though he certainly had obstinate principles. A political man, he caught himself in a complex web of dissension between church and king. One contemporary described him as having ''burned with zeal for justice, but whether according to wisdom, God knows.''
Nevertheless, the pilgrims turned his veneration into a cult. And the Benedictine monks reaped great financial rewards. Much of this was fed back into the church building and its growing splendor.
The prominent placing of Becket's shrine explains to today's tourist something of the internal logic of the building. I suddenly visualized it, as Mrs. Ellender talked, as a setting for processions.
It is processional architecture. The pilgrims must have surged from west to east, and back again. She pointed out that the symbolic pattern in many churches is from old west to new east. ''The western half represents the Old Testament. The windows round the church tell the story of the Bible. The eastern half is the New Testament . . . the new dispensation coming in at the central crossing. . . . Then the story of Christ's life culminates in the eastern windows - the last days of his life on earth are told here. . . . But we also have Becket at Canterbury. So the eastern Corona chapel has a twofold purpose, and the glass on either side there is devoted to Becket.''
Emphasizing and dramatizing this procession from west to east even more, the cathedral floor rises, by a se-ries of flights of steps, toward the choir and eastern chapels. For the pilgrims this must have signified the gradual rising climax of their arrival at Becket's shrine.
When the two Williams' work was finished, building stopped almost completely at Canterbury. ''I think,'' says Mrs. Ellender with a chuckle, ''that everyone must have been very glad! They'd been at it for so long. Really, they did not start building again until the beginning of what we now think of as the 'perpendicular' style.''
This 14th-century style is seen in Canterbury's great nave, down at the western end. ''It is of course one of the finest, most beautiful naves - perpendicular at its best.''
We go and look.
It certainly is remarkably fine, tall and elegant, and unified. It is also surprisingly short. Henry Yevele, the master mason who designed it, was in fact hampered by the dimensions of the Norman nave his worked replaced. Begun in 1378 , his new nave (they still call it the ''new work'' here), with its slender avenue of piers climbing upward, interrupted only by unemphatic shaft-rings, and then springing high up into the vaulting above, was finally completed in 1405.
At this point, time began to figure in our visit to Canterbury. We had a train to catch midafternoon. So Mrs. Ellender hurried us to the northwest transept, the place of Becket's murder - a bare corner of shiny stone pavement - and from here down into the undercroft, or crypts.
Time presses as we move outdoors again for an all-too-brief look at the remains of the monastery on the cathedral's north side. We stand in what was the kitchen garden, and our inexhaustible guide points to the place where there were dormitories, kitchen - and to a complete and perfect gem of a water tower from the Norman period. Then - ''Remember I spoke inside about the pair of little Norman towers left standing when William of Sens did his rebuilding? There they are!'' (Her enthusiasm seems to climb with the architecture.)
She speaks in urgent italics now: ''Have you got enough time to walk over there or not? It's worth it. Run!'' - and we are careening wildly and unexpectedly in a northerly direction away from the cathedral. . . .
Breathless, I admit she was right: From this distance the entire splendor of this long cathedral is spread out before you. All its parts subordinate to the whole effect, grand and extensive.
''Oh, yes. Wonderful!'' I said. ''We'll remember our visit to Canterbury.''
''You can write and tell them that,'' she laughed lightly.
That view from the north, across a wide expanse of lawn, is what most stayed in the mind afterward. A magnificent array of glorious architecture - Norman, Early Gothic, Perpendicular. By no means simple, but a great diversity and complexity that embody a lengthy and various history.