"In days of old when knights were bold,

And suits were made of tin,

No mortal cry escaped the man Who sat upon a pin."

It would take more than the sturdiest of six-inch nails to pierce the massive armour of the Middle Ages. Ranging from the elaborately-damascened, short-skirted suit worn by the future Emperor, Charles V, as a boy, to the shining edifice built to protect the uncommonly large person of King Francis I, the collection displayed at the Vienna Kunsthistoriches Museum confirms the amazing skill of the medieval smiths, and together with metals of less belligerent purpose - including the great ornate collar of the Order of the Golden Fleece - forms part of a vast exhibition mounted to mark the 500th anniversary of the birth of Charles V.

He first saw the light in Ghent, and to this day is recalled in that city with a mixture of respect and loathing by descendants of the burghers he clobbered so ferociously in 1540. A captivating gouache by Jan Cornelisz presents the enthroned emperor as a mild toytown figure, but there is no mistaking the apprehension on the faces of the townspeople as they kneel before him, awaiting sentence for misguided rebellion.

In all, more than 400 artefacts have been assembled in Vienna to illustrate the life and times of Charles V. It was an era which saw the birth of both the Reformation and the Renaissance, accompanied by spectacular advances in the sciences, in navigation and in the arts, and all of them have a place at the exhibition. Inevitably however, its core remains the pictures which trace the rise of the Hapsburg family to mastery of the first empire on which "the sun never set."

Emperor Charles was the result of the Habsburg's policy of territorial expansion through judicious marriage. It was Charles's grandfather, Emperor Maximilian I, who made the boldest leap and brought Spain under Hapsburg domination by marrying his son, Philip, to the daughter of Ferdinand and Isabella.

Durer's portrait expresses Emperor Maximilian's gravity and wisdom with superb drawing and brushwork, while Bernard Strigel's lesser and highly sentimental study of the whole family imparts a benign piety to each of them. By contrast paintings of the Catholic Majesties of Spain introduce a pair of exceptionally dour countenances - possibly justified by acquaintance with their new son-in-law.

Despite his reputation as dissolute roisterer and womaniser, "Philip the Fair" looks incongruously lugubrious in the likeness made of him by John of Flanders. But there is no equivocation about the dangerous expression in the eyes of the companion portrait of his wife, "Joanna the Mad". Her husband's proclivities were not easily tolerated in the court of Toledo. As Grand Iquisitor, Cardinal Archbishop Cisneros's position there was impregnable, and it is clear from the delineation of his daunting profile in a striking alabaster relief by Felipe Bigarny and Fernando Ricon that he went at his work with a will.

No doubt "broken down by drink and sex", the fair Philip died young, his demented widow was placed under restraint, and his eldest son, Charles, was sent to Malines, to be brought up with his sister by his aunt, Archduchess Margaret of Austria. The tranquil sagacity that enabled this astute woman to achieve an interval in the wars with France by concluding the "Peace of the Ladies" with the mother of the French king are indicated in an anonymous wood painting, and again in a bust and medallion by Conrad Meit.

Margaret proved an exemplary guardian for young Charles, and while his personal preference was for hunting and war games, the boy was rigorously schooled in the art of government. Art works of the period give prominence to the salient Hapsburg facial characteristic of jutting jaw forcing the lower teeth forward, and this was even more pronounced in Charles's brother, Ferdinand, who is often unkindly depicted with his mouth hanging open. In time the assumption of beard and whiskers helped disguise the malformation, and later on, Charles's son, Philip II of Spain sports similar hairy camouflage in portraits by Titian and Alonso Sancho Coello.

Charles attained his majority at the tender age of15, and within five years was declared Holy Roman Emperor-Elect, defeating his hostile rival, Francis I of France, and also his uncle-by-marriage, Henry VIII. The event was duly celebrated by every sympathetic artist with a brush to wield, but by far the finest picture of the era has to be Albrecht Durer's study of the Jewish banker, Jacob Fugger, who contributed generously to Charles's election expenses. Then as now, votes were up for bids, and as the years passed, the Emperor came to depend on the Fuggers for war subsidies as well. An entire room is devoted to the family at the Exhibition, with pride of place held by Jorg Breu's huge and vivid oil-paintings describing the course of the seasons.

As Holy Roman Emperor, Charles V commanded the services of countless sculptors, artisans and painters. The pomp and circumstances of great cavalcades, important meetings, council and historic Diets are lavishly documented, but the work of Titian dominates the lot.

His much-reproduced portrait of Isabella of Portugal sets the pale, austere and beautiful features of the Emperor's wife at odds with her sumptuous and complicated costume of silks and velvet (inevitably inviting speculation about how long it took her to get dressed), and his love of fine fabrics is again demonstrated to nearly tangible effect in the rich and supple robes worn by Pope Paul III. This is one of the most thoughtful of his studies, displaying profound insight into the nature of its subjects.

Titian's influence on the art of his time was overwhelming, and is seen at its faithful best in an unattributed Venetian portrait of the Sultan, Suleiman the Magnificent. The face of Europe's arch-enemy is clear-cut, stern and handsome, though to modern eyes its unquestionable dignity is sadly, even comically, impaired by the giant white turban that rises above it like an outsize pumpkin.

If the mighty Ottoman posed a threat to Christendom as a whole, Martin Luther proved a personal and perennial thorn in the flesh of Charles V, effectively putting paid to the latter's ambition to unite Europe under the Hapsburg banners. It is in paintings of the leaders of the Reformation that Lucas Cranach the Elder comes into his own at the Vienna exhibition, and two likenesses of the monk as a young man already express his extreme seriousness of purpose. One is a wood-carving and the other an oil, and in both the face also reveals evidence of the discomfort engendered by the intestinal disorder from which Luther suffered throughout his life.

He has a more cheerful aspect in an engraving of his head made in 1521, with a healthier profile surmounted by large doctrinal hat. But a later portrait from Cranach's workshop shows the Reformer gloomy again, as always in deadly earnest, and now grown uncomfortably corpulent.

With the exception of Titian's shining portrait of Maurice of Saxony - painted perhaps before he changed sides - the Reformers are presented in sombre shades, providing a solemn counterpart to the exuberant colours favoured by the Empire and its artists. These last are at their most impressive in the many maps and tapestries His Imperial Majesty carried about with him on the ceaseless travelling necessitated by the care of several kingdoms. Naturally, their colours have faded over the centuries, but by the exercise of a strange alchemy, the photographs in the exhibition catalogue review much of their original brilliance, almost defying belief in the breath-taking imagination and detail that makes up their complex design.

The Kaiser Karl V Exhibition continues at the Kunsthistoriches Museum in Vienna until September 10th